Problem:
Greatest Common Divisor

Write a program which finds the greatest common divisor of two natural numbers a and b

Input

a and b are given in a line sparated by a single space.

Output

Output the greatest common divisor of a and b.

Constrants

1 ≤ a, b ≤ 109

Hint

You can use the following observation:

For integers x and y, if x ≥ y, then gcd(x, y) = gcd(y, x%y)

Sample Input 1

54 20

Sample Output 1

2

Sample Input 2

147 105

Sample Output 2

21

Solution in C# (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
//using System.Collections;
using System.Linq;

namespace solution
{
  class Class1
  {
    static void Main()
    {
      var num = Console.ReadLine().Split().Select(decimal.Parse).ToArray();
      var n0 = new List<decimal>();
      var n1 = new List<decimal>();

      for (int i = 1; i <= num[0]; i++)
      {
        if (num[0] % i == 0) n0.Add(i);
      }

      for (int i = 1; i <= num[1]; i++)
      {
        if (num[1] % i == 0) n1.Add(i);
      }

      var seki = (n0.Intersect(n1));
      Console.WriteLine(seki.Max());
    }
  }
}