#include<bits/stdc++.h>
using namespace std;
int main() {
    int x, y, n, p;
    cin >> x >> y >> n >> p;
double a, b; 
    if (p >= x) {
        a = p - y;
    } else {
        a = p; 
    }
    b = p * n / 10.0; 
    cout << fixed << setprecision(2) << min(a, b);
return 0;
}