#include<iostream>
using namespace std;
int c[370];
int main() {
	int a, b, m, n, i, j, k, sum = 0;
	cin >> a >> b >> m >> n;
	c[1] = a;
	c[2] = b;
	for (i = 3;; i++) {
		c[i] = c[i - 1] + c[i - 2];
		if (c[i] >= m) break;
	}
	for (i = 1; i <= n; i++) {
		sum = sum + c[i];
	}
	cout << sum << endl;
	return 0;
}