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

int main() {
	int a, b, m, n;
	int box[10000];
	int ans = 0;
	cin >> a >> b >> m >> n;
	box[0] = a;
	box[1] = b;

	for (int i = 0; i < n; i++) {
		if (i >= 3) {
			box[i] = box[i - 1] + box[i - 2];
		}
		ans += box[i];
		if (box[i] >= m) {
			break;
		}
	}

	ans += a;
	ans += b;
	cout << ans;
	return 0;
}