using System;

using System.Numerics;

​

public class Program

{

    public static void Main()

    {

        BigInteger t1 = BigInteger.Parse(Console.ReadLine());

        BigInteger t2 = BigInteger.Parse(Console.ReadLine());

        BigInteger t3 = BigInteger.Parse(Console.ReadLine());

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

​

        if (n == 1)

        {

            Console.WriteLine(t1);

        }

        else if (n == 2)

        {

            Console.WriteLine(t2);

        }

        else if (n == 3)

        {

            Console.WriteLine(t3);

        }

​

        else

        {

            BigInteger nextElement = 0;

​

            for (int i = 3; i <= n - 1; i++)

            {

                nextElement = t1 + t2 + t3;

                t1 = t2;

                t2 = t3;

                t3 = nextElement;

            }

            Console.WriteLine(nextElement);

        }

    }

}