diff --git a/du1/program.c b/du1/program.c
new file mode 100644
index 0000000..2f76fbd
--- /dev/null
+++ b/du1/program.c
@@ -0,0 +1,19 @@
+#include <stdio.h>
+
+int main(){
+    while(1){
+        char ch;
+        int num = 'a' - 'A';
+        scanf("%c", &ch);
+        if(ch == EOF || ch == -1){
+            return 0;
+        }
+        if(ch <= 'A' && ch >= 'Z'){
+            ch += num;
+        }
+        else if(ch >= 'a' && ch <= 'z'){
+            ch -= num;
+        }
+        printf("%c", ch);
+    }
+}