//
// Created by Nitit Jongsawatsataporn on 10/1/23.
//
#include<iostream>
using namespace std;

double h,r,a,w;

double cm(double wh) {
    double air_mass = (h-wh)*r*r*a;
    double wa_mass = wh*r*r*w;
    double cm = (air_mass * (wh + (h-wh)/2) + wa_mass * (wh/2))/(air_mass + wa_mass);
    return cm;
}

int main() {
    cin>>h>>r>>a>>w;
    double l,r,ll,rr;
    l = 0, r = h;
    for(int i=0;i<100000;i++) {
        ll = (2*l+r)/3;
        rr = (2*r+l)/3;
        if(cm(ll) > cm(rr)) {
            l = ll;
        }
        else {
            r = rr;
        }
    }
    printf("%.8f",l);
    return 0;
}