Problem 7.Calculate N! / (K! * (N-K)!)

using System;
using System.Numerics;

class numberOfCombinations
{
    static void Main()
    {
        BigInteger n = BigInteger.Parse(Console.ReadLine());
        BigInteger k = long.Parse(Console.ReadLine());

        BigInteger nFactoriel = 1;
        BigInteger kFactoriel = 1;
        BigInteger nkFactoriel = 1;
        if (n > 1 && n < 100 && k > 1 && k < 100 && k < n)
        {
            for (int i = 0; i < n; i++)
            {
                nFactoriel *= n - i;
                if (k-i > 0)
                {
                    kFactoriel *= k - i;
                }
                if (n-k-i > 0)
                {
                    nkFactoriel *= (n - k) - i;
                }
            }
        }
        Console.WriteLine(nFactoriel / (kFactoriel * nkFactoriel));
    }
}