#include<iostream>
#include<string>
using namespace std;
int main() {
    int A;
    int B;
    int L;
    int R;
    cin >> L >> R >> A >> B;
    int ans = 0;
    if (A < 0) {
        ans = A * L + B;
        cout << ans;
    }
    else {
        ans = A * R + B;
        cout << ans;
    }
}