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;

namespace ALDS1_1_B
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] d = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int a = d[0], b = d[1];
            while (true)
            {
                if (a == 0)
                {
                    Console.WriteLine(b);
                    break;
                }
                if (b == 0) { Console.WriteLine(a); break; }
                if (a > b) { a = a - b; }
                else { b = b - a; }
            }
        }
    }
}