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 a = int.Parse(input[0]);
            int b = int.Parse(input[1]);
            if (a < b) {
                int temp = a;
                a = b;
                b = temp;
            }

            int r = a % b;
            while (r != 0) {
                a = b;
                b = r;
                r = a % b;
            }
            Console.WriteLine(b);
        }
    }
}