Tribonacci

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

namespace Tribonacci
{
    class Tribonacci
    {
        static void Main(string[] args)
        {
            BigInteger firstNumber = BigInteger.Parse(Console.ReadLine());
            BigInteger secondNumber = BigInteger.Parse(Console.ReadLine());
            BigInteger thirdNumber = BigInteger.Parse(Console.ReadLine());

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

            if (numberN == 1)
            {
                Console.WriteLine(firstNumber);
            }
            else if (numberN == 2)
            {
                Console.WriteLine(secondNumber);
            }
            else if (numberN == 3)
            {
                Console.WriteLine(thirdNumber);
            }
            else
            {
                for (int i = 4; i <= numberN; i++)
                {
                    BigInteger nextNumber = thirdNumber + secondNumber + firstNumber;
                    firstNumber = secondNumber;
                    secondNumber = thirdNumber;
                    thirdNumber = nextNumber;
                }
                Console.WriteLine(thirdNumber);
            }

        }
    }
}