#include<bits/stdc++.h>
#define rep(i,a,n) for (int i = a;i < n;i++)
using namespace std;

int main() {
    double a, b, x, y;
    cin >> a >> b >> x >> y;
    if (x / a * b <= y){
        printf("%f", x + x / a * b);
    }else{
        printf("%f", y + y / b * a);
    }
}