Calculate GCD

using System;

class CalculateGCD
{
    static void Main()
    {
        Console.Write("a = ");
        int firstNumber = int.Parse(Console.ReadLine());
        Console.Write("b = ");
        int secondNumber = int.Parse(Console.ReadLine());

        int greatestCommonDivisor = 1;

        for (int i = 1; i <= Math.Min(Math.Abs(firstNumber), Math.Abs(secondNumber)); i++)
        {
            if (firstNumber % i == 0 && secondNumber % i == 0)
            {
                greatestCommonDivisor = i;
            }
        }

        Console.WriteLine();
        Console.WriteLine("GCD({0}, {1}) = {2}\r\n", firstNumber, secondNumber, greatestCommonDivisor);
    }
}