#include <bits/stdc++.h>
using namespace std;

int a, b, m, N, cnt, num[2], tmp;

int main() {
	cin >> a >> b >> m >> N;
	num[1] = a;
	num[2] = b;
	while (N && num[1] < m) {
		cnt+=num[1];
		tmp = num[1];
		num[1] = num[2];
		num[2]+=tmp;
		N--;
	}
	cout << cnt;
	return 0;
}