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

public class Tribonacci {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		BigInteger a = sc.nextBigInteger();
		BigInteger b = sc.nextBigInteger();
		BigInteger c = sc.nextBigInteger();
		int n = sc.nextInt();
		BigInteger tribNumber = BigInteger.valueOf(0);
		
		if (n==1) {
			System.out.println(a);
		} else if (n==2) {
			System.out.println(b);
		} else if (n==3) {
			System.out.println(c);
		} else{

			for (int i = 3; i < n; i++) {
				tribNumber = a.add(b).add(c);
				a = b;
				b = c;
				c = tribNumber;
			}
			System.out.println(tribNumber);
		}
	}
}