Tribonachi

import java.math.BigInteger;
import java.util.Scanner;

import sun.rmi.runtime.NewThreadAction;

public class tribonachi
{
	public static void main(String[] args)
	{
		Scanner scan = new Scanner(System.in);
		BigInteger first = scan.nextBigInteger();

		scan.nextLine();
		BigInteger second = scan.nextBigInteger();

		scan.nextLine();
		BigInteger third = scan.nextBigInteger();

		scan.nextLine();
		int n = scan.nextInt();
		BigInteger tN = new BigInteger("0");

		for (int i = 0; i < n - 3; i++)
		{
			tN = first.add(second);
			tN = tN.add(third);
			BigInteger temp = tN;
			first = second;
			second = third;
			third = temp;
		}

		System.out.println(tN);
	}
}