CombinationFormula

using System;
using System.Numerics;

class CombinationFormula
{
    static void Main()
    {
        Console.Write("n = ");
        int n = int.Parse(Console.ReadLine());
        Console.Write("k = ");
        int k = int.Parse(Console.ReadLine());

        if (n > 1 && k > 1 && n < 100 && k < 100)
        {
            BigInteger factN = 1;
            BigInteger factK = 1;
            BigInteger factNAndK = 1;
            BigInteger result = 0;

            for (int i = 1; i <= n; i++)
            {
                factN *= i;
            }

            for (int j = 1; j <= k; j++)
            {
                factK *= j;
            }

            for (int z = 1; z <= n - k; z++)
            {
                factNAndK *= z;
            }

            result = factN / (factK * factNAndK);

            Console.WriteLine(result);
        }
        else
        {
            Console.WriteLine("Invalid input!");
        }
    }
}