#include "iostream"  
    #include "cmath"  
    using namespace std;  
    //一个数和0的最大公约数就是这个数本身  
    //a / b = q + r => gcd(a,b) = gcd(b,r)  
    int gcd(int a, int b){  
        int r = a % b; //余数  
        int q = a / b; //商''''''  
        if (r == 0)  
            return b;  
        else  
            return gcd(b,r);  
    }  
      
    int main(){  
      
        cout << gcd(4,18);  
        return 0;  
    }