Problem_2___Tribonacci

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

namespace Problem_2___Tribonacci
{
    class Program
    {
        static void Main(string[] args)
        {
            long t1 = long.Parse(Console.ReadLine());
            long t2 = long.Parse(Console.ReadLine());
            long t3 = long.Parse(Console.ReadLine());

            decimal n = decimal.Parse(Console.ReadLine());

            long tn = 0;
            for (int i = 4; i <= n; i++)
            {
                 tn = t1 + t2 + t3;
                t1 = t2;
                t2 = t3;
                t3 = tn;


            }

            Console.WriteLine(tn);

        }
    }
}