#include <bits/stdc++.h>
using namespace std;
int main(){
    double Vl,Vr,d,w;
    cin>>Vl>>Vr>>d>>w;
    printf("%.8f",2*d*w*w/(w+Vr)/(w+Vl)/(1-(w-Vl)*(w-Vr)/(w+Vl)/(w+Vr)));
}