Greatest Common Divisor (GCD)

/*
 * Write a program that calculates the greatest common divisor (GCD) of given two numbers. Use the Euclidean algorithm (find it in Internet).
*/

using System;
using System.Numerics;

class GreatestCommonDivisor
{
    static void Main()
    {
        BigInteger numberA, numberB;

        string invalidInput = "Please enter a valid number greater than 0!" + Environment.NewLine;

        Console.WriteLine("Enter a value for number A: ");
        while (!(BigInteger.TryParse(Console.ReadLine(), out numberA) && numberA > 0))
        {
            Console.WriteLine(invalidInput);
            Console.WriteLine("Enter a value for number A: ");
        }

        Console.WriteLine("Enter a value for number B: ");
        while (!(BigInteger.TryParse(Console.ReadLine(), out numberB) && numberB > 0))
        {
            Console.WriteLine(invalidInput);
            Console.WriteLine("Enter a value for number B: ");
        }

        while (numberA != numberB)
        {
            if (numberA > numberB)
            {
                numberA = numberA - numberB;
            }
            else
            {
                numberB = numberB - numberA;
            }
        }
        Console.WriteLine("The greatest common divisior (GSD) for numbers {0} and {1} is {2}." + Environment.NewLine, numberA, numberB, numberA);
        Main();
    }
}