int gcd (int u, int v) { /* calculate the gcd of u and v */
    if (v == 0) return u;
    else return gcd(v, u - u / v * v); /* v,u-u/v*v is equals to u mod v*/
}
int main(void) {
        int x; int y; int temp;
    x = 72;
        y = 18;
    if (x<y) {
        temp = x;
        x = y;
        y = temp;
    }
    gcd(x,y);
    return 0;
}