using System;
using System.Numerics;

class Tribonacci
{
    static void Main()
    {

        BigInteger a = long.Parse(Console.ReadLine());
        BigInteger b = long.Parse(Console.ReadLine());
        BigInteger c = long.Parse(Console.ReadLine());
        int n = int.Parse(Console.ReadLine());
        int counter = n - 3;
        BigInteger member = 0;
        //---------------------------------------

        switch (counter)
        {
            case 0: Console.WriteLine(c); break;
            case -1: Console.WriteLine(b); break;
            case -2: Console.WriteLine(a); break;
            default:

                do
                {
                    if (true)
                    {
                        if (counter < 1)
                        {
                            break;
                        }
                        counter--;
                        a = a + b + c;
                        member = a;
                    }

                    if (true)
                    {
                        if (counter < 1)
                        {
                            break;
                        }
                        counter--;
                        b = a + b + c;
                        member = b;
                    }

                    if (true)
                    {
                        if (counter < 1)
                        {

                            break;
                        }
                        counter--;
                        c = a + b + c;
                        member = c;
                    }

                } while (counter > 0);
                //---------------------------------------

                Console.WriteLine(member); break;
        }
    }
}