Calculation With Factorials

/*
 * Write a program that calculates N!*K! / (K-N)! for given N and K (1<N<K).
*/

using System;
using System.Numerics;

class CalculationWithFactorials
{
    static void Main()
    {
        BigInteger numberN, numberK, result;
        string invalidInput = "Please enter a valid number! (1 < N < K)" + Environment.NewLine;

        while (true)
        {
            BigInteger factorial = 1;

            Console.WriteLine("Enter a value for K: ");
            while (!(BigInteger.TryParse(Console.ReadLine(), out numberK) && numberK > 2))
            {
                Console.WriteLine(invalidInput);
                Console.WriteLine("Enter a value for K: ");
            }

            Console.WriteLine("Enter a value for N: ");
            while (!(BigInteger.TryParse(Console.ReadLine(), out numberN) && numberN > 1 && numberN < numberK))
            {
                Console.WriteLine(invalidInput);
                Console.WriteLine("Enter a value for N: ");
            }

            BigInteger differenceKN = numberK - numberN;

            for (BigInteger i = 2; i <= differenceKN; i++)
            {
                factorial = factorial * i;
            }
            BigInteger factorialKN = factorial;
            if (numberN != differenceKN)
            {
                for (BigInteger i = differenceKN + 1; i <= numberN; i++)
                {
                    factorial = factorial * i;
                }
            }
            else
            {
                factorial = factorialKN;
            }

            factorial = factorial * factorial;
            for (BigInteger i = numberN + 1; i <= numberK; i++)
            {
                factorial = factorial * i;
            }
            result = factorial / factorialKN;
            Console.WriteLine("{0}! * {1}! / ({0} - {1})! = {2}" + Environment.NewLine, numberK, numberN, result);
        }
    }
}