C++ :

#include<cstdio>
#include<math.h>
int main()
{
	int x,y,r;
	scanf("%d %d",&x,&y);
	if(y>x)
	{
		r=y;
		y=x;
		x=r;
	}
	r=x%y;
	while(r!=0)
	{
		x=y;
		y=r;
		r=x%y;
	}
	printf("%d",y);
}

Pascal :

var
 m,n,a,b,r:integer;
begin
 readln(m,n);
 a:=m;b:=n; r:=a mod b;
 while  r<>0 do
 begin
  a:=b;
  b:=r;
  r:=a mod b;
 end;

 write(b);
end.