C :

#include<stdio.h> 
int Gcd(int a,int b); 
int main() 
{ 
    int m,n; 
    scanf("%d,%d",&m,&n); 
    printf("%d\n",Gcd(m,n)); 
    return 0; 
} 
int Gcd(int a,int b) 
{
	if(a<=0||b<=0) return -1;
	while(a!=b)
	{
		if(a>b)
	        a=a-b;
	    else if(b>a)
	        b=b-a;
	}
	return a;
} 

C++ :

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int way(int a,int b)
{
	if (a>b)
	return way(a-b,b);
	else if(a<b) return(way(a,b-a));
	else return a;
}
int main()
{
	int n,m;
	cin>>n;
	getchar();
	cin>>m;
	cout<<way(n,m);
	return 0;
}