C++ :

#include<iostream>
using namespace std;
int main()
{
	int m,n,y;
	cin>>m>>n;
	while (n!=0)
	{
		y=m%n;
		m=n;
		n=y;
	}
	cout<<m;
	return 0;
}

Pascal :

var m,n,y,b,c:integer;
begin
 readln(m,n);
 b:=m;c:=n;
 repeat
  y:=b mod c;
  b:=c;
  c:=y;
 until y=0;
 writeln(b);
end.