#include <bits/stdc++.h>

using namespace std;

int main(){
    int a,b,x,y;cin>>a>>b>>x>>y;
    if(a*y<=b*x){
        cout<<fixed<<setprecision(7)<<(double)y*(a+b)/b<<endl;
    }else{
        cout<<fixed<<setprecision(7)<<(double)y*(a+b)/b<<endl;
    }
    return 0;
}