05_CalculateNFaktMultiplyKFact

using System;
using System.Numerics;

/*
 * 5. Write a program that calculates N!*K! / (K-N)! for given N and K (1 < N < K).
 */
class CalculateNFaktMultiplyKFact
{
    static void Main()
    {
        uint n;
        uint k;
        BigInteger result = 1;

        Console.Write("N = ");
        string strN = Console.ReadLine();

        Console.Write("K = ");
        string strK = Console.ReadLine();

        if (!uint.TryParse(strN, out n))
        {
            Console.WriteLine("Invalid number: {0}", strN);
        }
        else if (!uint.TryParse(strK, out k))
        {
            Console.WriteLine("Invalid number: {0}", strK);
        }
        else
        {
            if (k > n && n > 1)
            {
                for (uint i = 1; i <= k; i++)
                {
                    if (i <= n)
                    {
                        result *= i;
                    }

                    if (i >= (k - n + 1))
                    {
                        result *= i;
                    }
                }

                Console.WriteLine("The result is: {0}", result);
            }
            else
            {
                Console.WriteLine("Wrong numbers entered!");
            }
        }
    }
}