H6_Loops_T5_FactorialNK

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

using System;
using System.Numerics;

class NKfactorialDividedKMinusNFactorial
{
    static void Main()
    {
        Console.WriteLine("Please enter number n, which has to be integer bigger than 1:");
        int n = int.Parse(Console.ReadLine());
        Console.WriteLine("Please enter number k, which has to be integer bigger than n:");
        int k = int.Parse(Console.ReadLine());

        BigInteger productNFactorial = 1;
        if (n > 1 && k > n)//Checks input data.
        {
            for (int i = n; i > 1; i--)
            {
                productNFactorial = productNFactorial * i;//Calculates N!
            }


            BigInteger productKFactorialDividedKMunusNFactorial = 1;
            for (int i = k; i > (k - n); i--)// Calculates K! / (K-N)!, but omits the common parts.
            {
                productKFactorialDividedKMunusNFactorial = productKFactorialDividedKMunusNFactorial * i;
            }
            BigInteger finalResult = productKFactorialDividedKMunusNFactorial * productNFactorial;
            Console.WriteLine("The result is: {0}", finalResult);
        }
        else
        {
            Console.WriteLine("Wrong input!");
        }
    }
}