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.Linq;
using System.Text.RegularExpressions;

namespace CsProgramSpace
{
    public partial class Pencil
    {

        partial class Program
        {
            static void Main(string[] args)
            {
                long x, y;
                long[] a = Console.ReadLine().Split(' ').Select(_ => long.Parse(_)).ToArray();
                x = a[0];y = a[1];
                long mod;
                if(x>y)
                {
                    while(true)
                    {
                        
                        mod = x % y;
                        if (mod == 0)
                        {
                            break;
                        }
                        x = y;
                        y = mod;
                    }
                }
                else
                {
                    long t;
                    t = x;
                    x = y;
                    y = t;
                    while (true)
                    {
                        mod = x % y;
                        if (mod == 0)
                        {
                            break;
                        }
                        x = y;
                        y = mod;
                    }
                }
                Console.WriteLine(y);
            }
        }
    }
}