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;

namespace ALDS1_1_B
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] input = Console.ReadLine().Split(' ');
            int num1 = int.Parse(input[0]);
            int num2 = int.Parse(input[1]);
            int gcd = 0;

            if (num1 <= num2)
            { 
                gcd = calcu_gcd(num2, num1);
            }
            else
            {
                gcd = calcu_gcd(num1, num2);
            }

            Console.WriteLine(gcd);
        }

        static int calcu_gcd(int x, int y)
        {
            int tmp_x = x;
            int tmp_y = y;
            int div = 0;
            int check = 0;
            bool div_flg = false;

            while (div_flg == false)
            {
                div = tmp_x % tmp_y;
                if (div == 0)
                {
                    div = tmp_x;
                    break;
                }

                check = tmp_y % div;
                
                if (check == 0)
                {
                    div_flg = true;
                }
                else
                {
                    tmp_x = tmp_y;
                    tmp_y = div;
                }
            }

            return div;
        }
    }
}