Negative days completion.

programs/abs/info.json

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+{
+  "language": "C",
+  "name": "演習課題 2",
+  "description": "整数値を読み込み、その絶対値を出力するプログラム。",
+  "output": {
+    "type": "screenshot",
+    "content": "./assets/abs.png"
+  },
+  "note": ""
+}

programs/abs/main.c

@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main(void) {
+    int x, y;
+
+    printf("Input integer: ");
+    scanf("%d", &x);
+
+    y = x;
+
+    if (y < 0)
+        y = -y;
+    
+    printf("Absolute value of %d is %d.\n", x, y);
+
+    return 0;
+}