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

int f[21] = {0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765};

int main() {
	int a, n, m, x;
	cin >> a >> n >> m >> x;
	int ans = a;
	for (int i = 2; i <= x; i++) {
//		cout << f[i] << ' ';
		ans += (f[i] - f[i - 1]) * a;
	}
	cout << ans - (m % a);
}