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 num1 = int.Parse(Console.ReadLine());
            BigInteger num2 = int.Parse(Console.ReadLine());
            BigInteger num3 = int.Parse(Console.ReadLine());
            uint n = uint.Parse(Console.ReadLine());
            BigInteger nextNum = 0;
            if (n == 1)
            {
                nextNum = num1;
                Console.WriteLine(nextNum);
            }
            else if (n == 2)
            {
                nextNum =num2 ;
                Console.WriteLine(nextNum);
            }
            else if (n ==3)
            {
                nextNum = num3;
                Console.WriteLine(nextNum);
            }
            else
            {
                for (int i = 0; i < n - 3; i++)
                {
                    nextNum = num1 + num2 + num3;
                    num1 = num2;
                    num2 = num3;
                    num3 = nextNum;
                }
                Console.WriteLine(nextNum);
            }
        }
    }
}