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;
using static System.Console;
using static System.Math;

namespace Topic01
{
    class ALDS1_1_B
    {
        static void Main(string[] args)
        {
            int[] init = ReadLine().Split()
                .Select(n => int.Parse(n))
                .OrderByDescending(n => n).ToArray();
            
            int A = init[0];
            int B = init[1];
            int R = B;
            while (R > 0)
            {
                R = A % B;
                A = B;
                B = R;
            }

            int gcd = A;
            WriteLine(gcd.ToString());
            ReadKey();
        }
    }
}