辗转相除法：

#include <stdio.h>


int gcd(int, int);

int main() {
    int num1, num2;
    scanf("%d%d", &num1, &num2);
    
    printf("%d\n", gcd(num1, num2));
    
    return 0;
}

int gcd(int a, int b) {
    if(b == 0) {
        return a;
    }
    return gcd(b, a % b);
}