**Tribonacci

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;
class Tribonacci
    {
        static void Main(string[] args)
        {
            BigInteger firstN = BigInteger.Parse(Console.ReadLine());
            BigInteger secondN = BigInteger.Parse(Console.ReadLine());
            BigInteger thirdN = BigInteger.Parse(Console.ReadLine());
            BigInteger n = BigInteger.Parse(Console.ReadLine());

            if(n == 1)
            {
                Console.WriteLine(firstN);
            }
            else if(n == 2)
            {
                Console.WriteLine(secondN);
            }
            else if(n == 3)
            {
                Console.WriteLine(thirdN);
            }

            else
            {
                BigInteger result = 0;
                    for (int i = 3; i < n; i++)
                {
                    result = firstN +secondN+thirdN;
                    firstN = secondN;
                    secondN = thirdN;
                    thirdN = result;
                }
                Console.WriteLine(result);
            }


        }
    }