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.Linq;

namespace ALDS1_1_B
{
    class Program
    {
        static void Main(string[] args)
        {
            //入力
            var st = Console.ReadLine().Trim();
            var vals = st.Split(" ".ToCharArray(), StringSplitOptions.RemoveEmptyEntries).Select(e => int.Parse(e)).ToArray();

            var x = vals[0];
            var y = vals[1];
            if (x < y)
            {
                var z = y;
                y = x;
                x = z;
            }

            while (y > 0)
            {
                var r = x % y;
                x = y;
                y = r;
            }
            Console.WriteLine(x);
        }
    }
}