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