https://www.csie.ntu.edu.tw/~b98902112/cpp_and_algo/cpp02/euclidean_algorithm.html
#include<iostream>
using namespace std;
int gcd( int a, int b )
{
if( b==0 )
return a;
return f( b, a%b );
}
int main()
{
int a=31031, b=10013;
cout << gcd(a,b) << endl;
return 0;
}