diff --git a/algorithm/.idea/misc.xml b/algorithm/.idea/misc.xml
index cf9abe6..b95853c 100644
--- a/algorithm/.idea/misc.xml
+++ b/algorithm/.idea/misc.xml
@@ -1,4 +1,3 @@
-<?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="ProjectRootManager" version="2" languageLevel="JDK_20" default="true" project-jdk-name="openjdk-20" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
diff --git a/algorithm/src/BJ1009.java b/algorithm/src/BJ1009.java
new file mode 100644
index 0000000..9c4dbdf
--- /dev/null
+++ b/algorithm/src/BJ1009.java
@@ -0,0 +1,22 @@
+import java.util.Scanner;
+
+public class BJ1009 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        int t = in.nextInt(); //반복 횟수
+        int a, b, c;
+
+        for(int i = 0; i < t; i++) {
+            a = in.nextInt();
+            b = in.nextInt();
+            c = 1;
+
+            for(int j = 0; j < b; j++){
+                c = (c * a) % 10;
+                if (c == 0)
+                    c = 10;
+            }
+            System.out.println(c);
+        }
+    }
+}
diff --git a/algorithm/src/BJ1212.java b/algorithm/src/BJ1212.java
new file mode 100644
index 0000000..7f46cb8
--- /dev/null
+++ b/algorithm/src/BJ1212.java
@@ -0,0 +1,27 @@
+import java.util.Scanner;
+
+public class BJ1212 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        String a = in.nextLine();
+        StringBuilder b = new StringBuilder();
+//객체를 생성하는 것이 아니라 기존의 데이터에 더하는 방식을 사용하여 속도가 빠르다고 한다.
+        for (int i = 0; i < a.length(); i++){ //입력된 값의 크기만큼 반복
+            char c = a.charAt(i); //a의 i번째 글자에 차례에 순서대로 갑니다.
+            switch (c) { // b.append(3 "011", 1 "001", 4 "100") 더해줌
+                case '0': b.append("000"); break;
+                case '1': b.append("001"); break;
+                case '2': b.append("010"); break;
+                case '3': b.append("011"); break;
+                case '4': b.append("100"); break;
+                case '5': b.append("101"); break;
+                case '6': b.append("110"); break;
+                case '7': b.append("111"); break;
+                default: break;
+            }
+        }
+        if (b.charAt(0)=='0') b.deleteCharAt(0);
+        if (b.charAt(0)=='0') b.deleteCharAt(0);
+        System.out.println(b);
+    }
+}
diff --git a/algorithm/src/BJ1373.java b/algorithm/src/BJ1373.java
new file mode 100644
index 0000000..fb628e1
--- /dev/null
+++ b/algorithm/src/BJ1373.java
@@ -0,0 +1,29 @@
+import java.util.Scanner;
+public class BJ1373 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        String a = in.nextLine();
+        int size = a.length();
+        if(size%3==1)
+            a = "00" + a;
+        if(size%3==2)
+            a = "0" + a;
+
+        StringBuilder b = new StringBuilder();
+
+        int q = 0;
+        int w = 0;
+        int e;
+
+        for (int i = 0; i <= a.length()-1; i++){
+            int num = a.charAt(i)-'0';
+            if(i%3==0) q = num*4;
+            else if(i%3==1) w = num*2;
+            else {
+                e = num;
+                b.append(q+w+e);
+            }
+        }
+        System.out.println(b);
+    }
+}
\ No newline at end of file
diff --git a/algorithm/src/BJ1789.java b/algorithm/src/BJ1789.java
new file mode 100644
index 0000000..3379492
--- /dev/null
+++ b/algorithm/src/BJ1789.java
@@ -0,0 +1,18 @@
+import java.util.Scanner;
+
+public class BJ1789 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        long a = in.nextLong();
+        //자바 int는 21억정도까지 밖에 안됨
+        //그러나 문제는 42억까지 요구함
+        long sum = 0;
+        int count = 0;
+        for(int i = 1; ; i++) {
+            if(sum > a) break;
+            count++;
+            sum += i;
+        }
+        System.out.println(count-1);
+    }
+}
diff --git a/algorithm/src/BJ1934.java b/algorithm/src/BJ1934.java
new file mode 100644
index 0000000..3c4725f
--- /dev/null
+++ b/algorithm/src/BJ1934.java
@@ -0,0 +1,21 @@
+import java.util.Scanner;
+
+public class BJ1934 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        int t = in.nextInt();
+        int d = 0;
+        for (int i = 0; i < t; i++) {
+            int a = in.nextInt();
+            int b = in.nextInt();
+            d = a * b;
+            int c;
+            while (b != 0) {
+                c = a % b;
+                a = b;
+                b = c;
+            }
+            System.out.println(d/a);
+        }
+    }
+}
diff --git a/algorithm/src/BJ1978.java b/algorithm/src/BJ1978.java
new file mode 100644
index 0000000..5c5831f
--- /dev/null
+++ b/algorithm/src/BJ1978.java
@@ -0,0 +1,21 @@
+import java.util.Scanner;
+public class BJ1978 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        int N = in.nextInt();
+        int[] a = new int[N];
+        int sum = 0;
+        for (int i = 0; i < a.length; i++) {
+            a[i] = in.nextInt();
+            for(int j = 2; j <= a[i]; j++) {
+                if(j == a[i]){
+                    sum++;
+                }
+                if (a[i] % j == 0) {
+                    break;
+                }
+            }
+        }
+        System.out.println(sum);
+    }
+}
\ No newline at end of file
diff --git a/algorithm/src/BJ2839.java b/algorithm/src/BJ2839.java
new file mode 100644
index 0000000..8850e1f
--- /dev/null
+++ b/algorithm/src/BJ2839.java
@@ -0,0 +1,20 @@
+import java.util.Scanner;
+
+public class BJ2839 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        int N = in.nextInt();
+        int sum = 0;
+        while(true) {
+            if(N % 5 == 0) {
+                System.out.println(N / 5 + sum);
+                break;
+            } else if (N < 0){
+                System.out.println(-1);
+                break;
+            }
+            N = N - 3;
+            sum++;
+        }
+    }
+}
diff --git a/algorithm/src/BJ2921.java b/algorithm/src/BJ2921.java
new file mode 100644
index 0000000..fb5d645
--- /dev/null
+++ b/algorithm/src/BJ2921.java
@@ -0,0 +1,18 @@
+import java.util.Scanner;
+public class BJ2921 {
+    public static void main(String[] args) {
+        Scanner in = new Scanner(System.in);
+        int N = in.nextInt();
+        int a = 3;
+        int b = 6;
+        int m = 0;
+        int sum = 0;
+        for (int i = 0; i < N; i++) {
+            m = a;
+            sum += m;
+            a += b;
+            b += 3;
+        }
+        System.out.println(sum);
+    }
+}