FactorialsProblem7

using System;
using System.Numerics;

namespace Problem7
{
    class CalculateFactorials2
    {
        static void Main(string[] args)
        {
            BigInteger firstNumber = BigInteger.Parse(Console.ReadLine());
            BigInteger secondNumber = BigInteger.Parse(Console.ReadLine());
            BigInteger n = firstNumber;
            BigInteger k = secondNumber;
            BigInteger kFact = 1;
            BigInteger nFact = 1;
            BigInteger nkFact = 1;
            for (BigInteger i = n; i > 1; i--)
            {
                    nFact *= i;
                    kFact *= k;
                    if (k != 1)
                    {
                        k--;
                    }
                if (i <= (firstNumber - secondNumber))
                {
                    nkFact *= i;
                }
            }
            Console.WriteLine(nFact/(kFact*nkFact));
        }
    }
}