Tribonacci

using System;
using System.Collections.Generic;
using System.Numerics; //use for the BigInteger

class Program
{
    static void Main()
    {
        BigInteger t1 = BigInteger.Parse(Console.ReadLine());
        BigInteger t2 = BigInteger.Parse(Console.ReadLine());
        BigInteger t3 = BigInteger.Parse(Console.ReadLine());
        BigInteger n = BigInteger.Parse(Console.ReadLine());
        BigInteger final = 0;

        if (n==1)
        {
            final = t1;
        }
        else if (n == 2)
        {
            final = t2;
        }
        else if (n == 3)
        {
            final = t3;
        }
        for (BigInteger i = 0; i < n - 3; i++)
        {
            final = t1 + t2 + t3;
            t1 = t2;
            t2 = t3;
            t3 = final;
        }
        Console.WriteLine(final);
    }
}