problem 2 - tribonacci

using System;

    class Program
    {
        static void Main()
        {
            long t1 = long.Parse(Console.ReadLine());
            long t2 = long.Parse(Console.ReadLine());
            long t3 = long.Parse(Console.ReadLine());
            long n = long.Parse(Console.ReadLine());
            long tn = 0;
            long tn_3 = t1;
            long tn_2 = t2;
            long tn_1 = t3;

            for (int i = 4; i <= n; i++)
            {
                tn = tn_3 + tn_2 + tn_1;
                tn_3 = tn_2;
                tn_2 = tn_1;
                tn_1 = tn;
            }
            Console.WriteLine(tn);
        }
    }