Tribonacci

using System;
using System.Numerics;

class Tribonacci
{
    static void Main()
    {

        BigInteger tNminus1 = new BigInteger(int.Parse(Console.ReadLine()));
        BigInteger tNminus2 = new BigInteger(int.Parse(Console.ReadLine()));
        BigInteger tNminus3 = new BigInteger(int.Parse(Console.ReadLine()));

        int numN = int.Parse(Console.ReadLine());

        if (numN == 1)
        {
            Console.WriteLine(tNminus1);
        }
        else if (numN == 2)
        {
            Console.WriteLine(tNminus2);
        }
        else if (numN == 3)
        {
            Console.WriteLine(tNminus3);
        }
        else
        {
            for (int i = 4; i <= numN; i++)
            {
                BigInteger tribonacciNew = tNminus1 + tNminus2 + tNminus3;
                tNminus1 = tNminus2;
                tNminus2 = tNminus3;
                tNminus3 = tribonacciNew;
            }
            Console.WriteLine(tNminus3);
        }

    }
}