Working code but assignment wants me to use a loop

Assignment 5/main.c

@@ -0,0 +1,25 @@
+/*
+ * Student: S.K. Soekhlal
+ * Number: 4860632
+ * Assignment: 3.4
+ */ 
+
+#include<stdlib.h>
+#include<stdio.h>
+
+int gcd (int p, int q)
+{
+	int r;
+
+	if ((r = p % q) == 0)
+		return q;
+	else
+		return gcd (q, r);
+}
+
+int main(){
+    int a,b;
+    scanf("%d%d",&a,&b);
+    printf("%d\n",gcd(a,b));
+    return 0;
+}