02. Tribonacci (6 Dec 2011 Morning)

using System;
using System.Numerics;


class Tribonacci
{
    static void Main()
    {
        BigInteger firstElement, secondElement, thurtElement, numberN;
        firstElement = int.Parse(Console.ReadLine());
        secondElement = int.Parse(Console.ReadLine());
        thurtElement = int.Parse(Console.ReadLine());
        numberN = int.Parse(Console.ReadLine());
        BigInteger nextElement = 0;


        for (int i = 0; i < numberN-3; i++)
        {
            nextElement = firstElement + secondElement + thurtElement;
            firstElement = secondElement;
            secondElement = thurtElement;
            thurtElement = nextElement;
        }
        if (numberN == 1)
        {
            Console.WriteLine(firstElement);
        }
        else if (numberN == 2)
        {
            Console.WriteLine(secondElement);
        }
        else if (numberN == 3)
        {
            Console.WriteLine(thurtElement);
        }
        else
        {
            Console.WriteLine(nextElement);
        }

    }
}