Tribonacci

using System;
using System.Numerics;

namespace Tribonacci
{
    class Tribonacci
    {
        static void Main()
        {
            BigInteger firstTrib = BigInteger.Parse(Console.ReadLine());
            BigInteger secondTrib = BigInteger.Parse(Console.ReadLine());
            BigInteger thirdTrib = BigInteger.Parse(Console.ReadLine());
            int n = int.Parse(Console.ReadLine());
            BigInteger currentTrib = 0;

            if (n == 1)
            {
                Console.WriteLine(firstTrib);
            }
            else if (n == 2)
            {
                Console.WriteLine(secondTrib);
            }
            else if (n == 3)
            {
                Console.WriteLine(thirdTrib);
            }
            else
            {
                for (int i = 3; i < n; i++)
                {
                    currentTrib = firstTrib + secondTrib + thirdTrib;
                    firstTrib = secondTrib;
                    secondTrib = thirdTrib;
                    thirdTrib = currentTrib;
                }
                Console.WriteLine(currentTrib);
            }
        }
    }
}