Factorials.cs

using System;
using System.Numerics;

namespace ConsoleApplication18
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine("Enter N=");
            BigInteger n = BigInteger.Parse(Console.ReadLine());
            Console.WriteLine("Enter K=");
            BigInteger k = BigInteger.Parse(Console.ReadLine());
            BigInteger sumK = 1;
            BigInteger sumN = 1;
            BigInteger sumM = 1;
            if (1 < k && k < n && n < 100)
            {
                for (int i = 1; i <= n; i++)
                {
                    sumN *= i;
                }
                for (int j = 1; j <= k; j++)
                {
                    sumK *= j;
                }
                for (int m = 1; m <= (n - k); m++)
                {
                    sumM *= m;
                }
                BigInteger sumF = sumN / (sumK * sumM);
                Console.WriteLine("{0}", sumF);
            }
            else
            {
                Console.WriteLine("Wrong input");
            }
        }
    }
}