C++ :

#include<iostream>
using namespace std;
int main()
{
	int a,b,r;
	cin>>a>>b;
	do
	{
		r=a%b;
		a=b;
		b=r;
	}
	while(r!=0);
	cout<<a;
	return 0;
}

Java :

import java.util.Scanner;
public class Main {
	public static int gcd(int a,int b){
		int temp;
		while(b!=0){
			temp=a%b;
			a=b;
			b=temp;
		}
		return a;
	}
	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		int m=sc.nextInt();
		int n=sc.nextInt();
		System.out.println(gcd(m,n));
		sc.close();
	}
}

Python :

# coding=utf-8
m,n=input().split()
m=int(m)
n=int(n)
for i in range(m,0,-1):
    if n%i==0 and m%i==0:
        print (i)
        break