Tribonacci Number

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

namespace Simple_Loops
{
    class Simple_Loops
    {
        static void Main(string[] args)
        {
            BigInteger n0 = BigInteger.Parse(Console.ReadLine());
            BigInteger n1 = BigInteger.Parse(Console.ReadLine());
            BigInteger n2 = BigInteger.Parse(Console.ReadLine());
            BigInteger next = 0;
            int N = int.Parse(Console.ReadLine());
                for (int i = 0; i < N - 3; i++)
                {
                   next = n0 + n1 + n2;
                   n0 = n1;
                   n1 = n2;
                   n2 = next;
                }
            if (N == 1) Console.WriteLine(n0);
            else if (N == 2) Console.WriteLine(n1);
            else if (N == 3) Console.WriteLine(n2);
            else Console.WriteLine(next);
        }
    }
}