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: Accepted)

using System;
using System.Collections.Generic;
using System.Text;

class Program
{
    public void Proc()
    {
        string[] inpt = Console.ReadLine().Split(' ');
        int x = int.Parse(inpt[0]);
        int y = int.Parse(inpt[1]);

        List<int> yakusu = this.GetYakusu(x);
        yakusu.Sort();
        for(int i=yakusu.Count-1; i>=0; i--)
        {
            if(y % yakusu[i] == 0)
            {
                Console.WriteLine(yakusu[i]);
                return;
            }
        }
        Console.WriteLine("-1");
    }

    private List<int> GetYakusu(int src)
    {
        List<int> ret = new List<int>();
        int tmp = src;
        for(int i=1; i<=src; i++)
        {
            if(i>tmp)
            {
                break;
            }
            if(src % i == 0)
            {
                ret.Add(i);
                tmp = src / i;
                ret.Add(tmp);
            }
        }
        return ret;
    }

    static void Main(string[] args)

    {

        Program prg = new Program();
        prg.Proc();

    }
}