function gcd_two_numbers(x, y) {  
  if ((typeof x !== 'number') || (typeof y !== 'number'))   
    return false;  
  x = Math.abs(x);  
  y = Math.abs(y);  
  while(y) {  
    var t = y;  
    y = x % y;  
    x = t;  
  }  
  return x;  
}  
  
console.log(gcd_two_numbers(12, 13));  
console.log(gcd_two_numbers(9, 3)); 