diff --git a/049-modp.dfy b/049-modp.dfy
new file mode 100644
index 0000000..0613916
--- /dev/null
+++ b/049-modp.dfy
@@ -0,0 +1,22 @@
+function modp_rec(n: int, p: int): int
+  requires p > 0
+  requires n >= 0
+{
+    if n == 0 then 1 % p else (modp_rec(n - 1, p) * 2) % p
+}
+
+method modp(n: int, p: int) returns (r: int)
+  requires p > 0
+  requires n >= 0
+  ensures r == modp_rec(n, p)
+{
+  r := 1 % p;
+  var i := 0;
+  while i < n
+    invariant 0 <= i <= n
+    invariant r == modp_rec(i, p)
+  {
+    r := (r * 2) % p;
+    i := i + 1;
+  }
+}
\ No newline at end of file
diff --git a/073-smallest_change.dfy b/073-smallest_change.dfy
new file mode 100644
index 0000000..c033bc9
--- /dev/null
+++ b/073-smallest_change.dfy
@@ -0,0 +1,19 @@
+method smallest_change(s: seq<int>) returns (c: int)
+  ensures c == |set i {:trigger s[i]} | 0 <= i < |s| / 2 && s[i] != s[|s| - 1 - i]|
+{
+  ghost var pos := {}; 
+  c := 0;
+
+  var i := 0;
+  while i < |s| / 2
+    invariant 0 <= i <= |s| / 2
+    invariant pos == set j {:trigger s[j]} | 0 <= j < i && s[j] != s[|s| - 1 - j]
+    invariant c == |pos|
+  {
+    if s[i] != s[|s| - 1 - i] {
+      pos := pos + {i};
+      c := c + 1;
+    }
+    i := i + 1;
+  }
+}
\ No newline at end of file
diff --git a/081-numerical_letter_grade.dfy b/081-numerical_letter_grade.dfy
new file mode 100644
index 0000000..f9e893c
--- /dev/null
+++ b/081-numerical_letter_grade.dfy
@@ -0,0 +1,67 @@
+method numerical_letter_grade(grades: seq<real>) returns (letters: seq<string>)
+  requires forall i :: 0 <= i < |grades| ==> 0.0 <= grades[i] <= 4.0
+  ensures |letters| == |grades|
+  ensures forall i :: 0 <= i < |grades| && grades[i] == 4.0 ==> letters[i] == "A+"
+  ensures forall i :: 0 <= i < |grades| && grades[i] < 4.0 && grades[i] > 3.7 ==> letters[i] == "A"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 3.7 && grades[i] > 3.3 ==> letters[i] == "A-"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 3.3 && grades[i] > 3.0 ==> letters[i] == "B+"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 3.0 && grades[i] > 2.7 ==> letters[i] == "B"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 2.7 && grades[i] > 2.3 ==> letters[i] == "B-"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 2.3 && grades[i] > 2.0 ==> letters[i] == "C+"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 2.0 && grades[i] > 1.7 ==> letters[i] == "C"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 1.7 && grades[i] > 1.3 ==> letters[i] == "C-"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 1.3 && grades[i] > 1.0 ==> letters[i] == "D+"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 1.0 && grades[i] > 0.7 ==> letters[i] == "D"
+  ensures forall i :: 0 <= i < |grades| && grades[i] <= 0.7 && grades[i] > 0.0 ==> letters[i] == "D-"
+  ensures forall i :: 0 <= i < |grades| && grades[i] == 0.0 ==> letters[i] == "E"
+{
+  letters := [];
+  var i := 0;
+  while i < |grades|
+    invariant 0 <= i <= |grades|
+    invariant |letters| == i
+    invariant forall j :: 0 <= j < i && grades[j] == 4.0 ==> letters[j] == "A+"
+    invariant forall j :: 0 <= j < i && grades[j] < 4.0  && grades[j] > 3.7 ==> letters[j] == "A"
+    invariant forall j :: 0 <= j < i && grades[j] <= 3.7 && grades[j] > 3.3 ==> letters[j] == "A-"
+    invariant forall j :: 0 <= j < i && grades[j] <= 3.3 && grades[j] > 3.0 ==> letters[j] == "B+"
+    invariant forall j :: 0 <= j < i && grades[j] <= 3.0 && grades[j] > 2.7 ==> letters[j] == "B"
+    invariant forall j :: 0 <= j < i && grades[j] <= 2.7 && grades[j] > 2.3 ==> letters[j] == "B-"
+    invariant forall j :: 0 <= j < i && grades[j] <= 2.3 && grades[j] > 2.0 ==> letters[j] == "C+"
+    invariant forall j :: 0 <= j < i && grades[j] <= 2.0 && grades[j] > 1.7 ==> letters[j] == "C"
+    invariant forall j :: 0 <= j < i && grades[j] <= 1.7 && grades[j] > 1.3 ==> letters[j] == "C-"
+    invariant forall j :: 0 <= j < i && grades[j] <= 1.3 && grades[j] > 1.0 ==> letters[j] == "D+"
+    invariant forall j :: 0 <= j < i && grades[j] <= 1.0 && grades[j] > 0.7 ==> letters[j] == "D"
+    invariant forall j :: 0 <= j < i && grades[j] <= 0.7 && grades[j] > 0.0 ==> letters[j] == "D-"
+    invariant forall j :: 0 <= j < i && grades[j] == 0.0 ==> letters[j] == "E"
+  {
+    if grades[i] == 4.0 {
+      letters := letters + ["A+"];
+    } else if grades[i] > 3.7 {
+      letters := letters + ["A"];
+    } else if grades[i] > 3.3 {
+      letters := letters + ["A-"];
+    } else if grades[i] > 3.0 {
+      letters := letters + ["B+"];
+    } else if grades[i] > 2.7 {
+      letters := letters + ["B"];
+    } else if grades[i] > 2.3 {
+      letters := letters + ["B-"];
+    } else if grades[i] > 2.0 {
+      letters := letters + ["C+"];
+    } else if grades[i] > 1.7 {
+      letters := letters + ["C"];
+    } else if grades[i] > 1.3 {
+      letters := letters + ["C-"];
+    } else if grades[i] > 1.0 {
+      letters := letters + ["D+"];
+    } else if grades[i] > 0.7 {
+      letters := letters + ["D"];
+    } else if grades[i] > 0.0 {
+      letters := letters + ["D-"];
+    } else {
+      letters := letters + ["E"];
+    }
+
+    i := i + 1;
+  }
+}
diff --git a/089-encrypt.dfy b/089-encrypt.dfy
new file mode 100644
index 0000000..ed54b85
--- /dev/null
+++ b/089-encrypt.dfy
@@ -0,0 +1,30 @@
+function rot_sym(c: char): char
+  requires 'a' <= c <= 'z'
+  ensures 'a' <= rot_sym(c) <= 'z'
+{
+  var alph := c as int - 'a' as int;
+  ((alph + 2 * 2) % 26 + 'a' as int) as char
+}
+
+method encrypt(s: string) returns (r: string)
+  requires forall i :: 0 <= i < |s| ==> 'a' <= s[i] <= 'z'
+  ensures |r| == |s|
+  ensures forall i :: 0 <= i < |s| ==> r[i] == rot_sym(s[i])
+{
+  r := "";
+  var i := 0;
+  while i < |s|
+    invariant 0 <= i <= |s|
+    invariant |r| == i
+    invariant forall j :: 0 <= j < i ==> r[j] == rot_sym(s[j])
+  {
+    r := r + [rot_sym(s[i])];
+    i := i + 1;
+  }
+}
+
+method Main() {
+    var s := "asdfghjkl";
+    var r := encrypt(s);
+    assert r == "ewhjklnop";
+}
\ No newline at end of file
diff --git a/108-count_nums.dfy b/108-count_nums.dfy
new file mode 100644
index 0000000..7eec499
--- /dev/null
+++ b/108-count_nums.dfy
@@ -0,0 +1,32 @@
+method count_nums(s: seq<int>) returns (cnt: nat) 
+  ensures cnt == |set i | 0 <= i < |s| && digits_sum(s[i]) > 0|
+{
+  ghost var found := {}; 
+
+  cnt := 0;
+  var i := 0;
+  while i < |s|
+    invariant 0 <= i <= |s|
+    invariant found == set j | 0 <= j < i && digits_sum(s[j]) > 0
+    invariant cnt == |found|
+  {
+    if digits_sum(s[i]) > 0 {
+      found := found + {i};
+      cnt := cnt + 1;
+    }
+    i := i + 1;
+  }
+}
+
+function digits_sum(x: int): int
+  decreases abs(x)
+{
+  if abs(x) < 10 then x else x % 10 + digits_sum(x / 10)
+}
+
+function abs(x: int): int 
+  ensures abs(x) >= 0
+  ensures abs(x) == x || abs(x) == -x
+{
+  if x >= 0 then x else -x
+}
\ No newline at end of file
diff --git a/114-minSubArraySum.dfy b/114-minSubArraySum.dfy
new file mode 100644
index 0000000..93f3826
--- /dev/null
+++ b/114-minSubArraySum.dfy
@@ -0,0 +1,28 @@
+function Sum(a: seq<int>, s: int, t: int): int
+  requires 0 <= s <= t <= |a|
+{
+  if s == t then 0 else Sum(a, s, t-1) + a[t-1]
+}
+
+method minSubArraySum(a: seq<int>) returns (s: int)
+  ensures forall p,q :: 0 <= p <= q <= |a| ==> Sum(a, p, q) >= s
+  ensures exists k, m :: 0 <= k <= m <= |a| && s == Sum(a, k, m)
+{
+  var k, m := 0, 0;
+  s := 0; 
+  var n := 0;
+  var c, t := 0, 0;
+  while n < |a|
+    invariant 0 <= c <= n <= |a| && t == Sum(a, c, n)
+    invariant forall b :: 0 <= b <= n ==> Sum(a, b, n) >= Sum(a, c, n)
+    invariant 0 <= k <= m <= n && s == Sum(a, k, m)
+    invariant forall p,q :: 0 <= p <= q <= n ==> Sum(a, p, q) >= Sum(a, k, m)
+  {
+    t, n := t + a[n], n + 1;
+    if t > 0 {
+      c, t := n, 0;
+    } else if s > t {
+      k, m, s := c, n, t;
+    }
+  }
+}
diff --git a/128-prod_signs.dfy b/128-prod_signs.dfy
new file mode 100644
index 0000000..68e44f8
--- /dev/null
+++ b/128-prod_signs.dfy
@@ -0,0 +1,71 @@
+function abs(x: int): int 
+  ensures abs(x) >= 0
+  ensures abs(x) == x || abs(x) == -x
+  ensures x >= 0 ==> abs(x) == x
+  ensures x < 0 ==> abs(x) == -x
+{
+  if x >= 0 then x else -x
+}
+
+function sum_abs(s: seq<int>) : int 
+  ensures sum_abs(s) >= 0
+{
+  if |s| == 0 then 0 else abs(s[0]) + sum_abs(s[1..])
+}
+
+lemma sum_prop(s: seq<int>) 
+  requires |s| > 0
+  ensures sum_abs(s) == sum_abs(s[..|s| - 1]) + abs(s[|s| - 1])
+{
+  if (|s| > 1) {
+    assert (s[1..][..|s[1..]| - 1]) == s[1..|s| - 1];
+  }
+}
+
+method prod_signs(numbers: seq<int>) returns (s: int)
+  ensures abs(s) == sum_abs(numbers)
+  ensures |set i | 0 <= i < |numbers| && numbers[i] < 0| % 2 == 1 ==> s <= 0
+  ensures |set i | 0 <= i < |numbers| && numbers[i] < 0| % 2 == 0 ==> s >= 0
+{
+  s := 0;
+  var i := 0;
+  while (i < |numbers|)
+    invariant 0 <= i <= |numbers|
+    invariant s == sum_abs(numbers[..i])
+  {
+    s := s + abs(numbers[i]);
+    assert sum_abs(numbers[..i + 1]) == sum_abs(numbers[..i]) + abs(numbers[i]) by {
+      assert numbers[..i+1][..i] == numbers[..i];
+      sum_prop(numbers[..i + 1]); 
+    }
+    i := i + 1;
+  }
+
+  assert numbers == numbers[..|numbers|];
+
+  i := 0;
+  ghost var negatives := {};
+  var cnt := 0;
+  while (i < |numbers|)
+    invariant 0 <= i <= |numbers|
+    invariant cnt == |negatives|
+    invariant negatives == set j | 0 <= j < i && numbers[j] < 0
+  {
+    if (numbers[i] < 0) {
+      negatives := negatives + {i};
+      cnt := cnt + 1;
+    }
+    i := i + 1;
+  }
+  assert negatives == set i | 0 <= i < |numbers| && numbers[i] < 0;
+
+  if (cnt % 2 == 1) {
+    s := -s;
+  }
+}
+
+method Main() {
+  var nums := [1, 2, 2, -4];
+  var s := prod_signs(nums);
+  print s;
+}
\ No newline at end of file
diff --git a/README.md b/README.md
index f1c0526..5d28ac6 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,7 @@ Current status:
 - [x] 46. fib4
 - [ ] 47. median
 - [x] 48. is_palindrome
-- [ ] 49. modp
+- [x] 49. modp
 - [ ] 50. encode_shift
 - [x] 51. remove_vowels
 - [x] 52. below_threshold
@@ -75,7 +75,7 @@ Current status:
 - [x] 70. strange_sort_list
 - [ ] 71. ~triangle_area~ -- requires real square root
 - [x] 72. will_it_fly
-- [ ] 73. smallest_change
+- [x] 73. smallest_change
 - [x] 74. total_match
 - [x] 75. is_multiply_prime
 - [x] 76. is_simple_power
@@ -83,7 +83,7 @@ Current status:
 - [x] 78. hex_key
 - [ ] 79. decimal_to_binary
 - [x] 80. is_happy
-- [ ] 81. numerical_letter_grade
+- [x] 81. numerical_letter_grade
 - [x] 82. prime_length
 - [x] 83. starts_one_ends
 - [ ] 84. solve
@@ -91,7 +91,7 @@ Current status:
 - [ ] 86. anti_shuffle - complex strings
 - [ ] 87. get_row - sorting
 - [x] 88. sort_array - sorting
-- [ ] 89. encrypt
+- [x] 89. encrypt
 - [ ] 90. next_smallest - nullable
 - [ ] 91. is_bored - complex strings
 - [x] 92. any_int
@@ -110,7 +110,7 @@ Current status:
 - [x] 105. by_length - sorting
 - [x] 106. f
 - [ ] 107. even_odd_palindrome
-- [ ] 108. count_nums
+- [x] 108. count_nums
 - [ ] 109. move_one_ball
 - [x] 110. exchange
 - [ ] 111. histogram
@@ -130,7 +130,7 @@ Current status:
 - [ ] 125. split_words - complex strings
 - [x] 126. is_sorted
 - [x] 127. intersection
-- [ ] 128. prod_signs
+- [x] 128. prod_signs
 - [ ] 129. minPath
 - [x] 130. tri
 - [ ] 131. digits
diff --git a/REMAINING.md b/REMAINING.md
index 9a141ad..642b432 100644
--- a/REMAINING.md
+++ b/REMAINING.md
@@ -9,21 +9,16 @@ Current status:
 - [ ] 38. encode_cyclic
 - [ ] 44. change_base
 - [ ] 47. median
-- [ ] 49. modp
 - [ ] 50. encode_shift
 - [ ] 56. correct_bracketing
 - [ ] 61. correct_bracketing
 - [ ] 65. circular_shift
-- [ ] 73. smallest_change
 - [ ] 79. decimal_to_binary
-- [ ] 81. numerical_letter_grade
 - [ ] 84. solve
-- [ ] 89. encrypt
 - [ ] 90. next_smallest - nullable
 - [ ] 93. encode
 - [ ] 103. rounded_avg
 - [ ] 107. even_odd_palindrome
-- [ ] 108. count_nums
 - [ ] 109. move_one_ball
 - [ ] 111. histogram
 - [ ] 112. reverse_delete
@@ -32,7 +27,6 @@ Current status:
 - [ ] 119. match_parens
 - [ ] 121. solution - sum
 - [ ] 124. valid_date
-- [ ] 128. prod_signs
 - [ ] 129. minPath
 - [ ] 131. digits
 - [ ] 132. is_nested