#include<bits/stdc++.h>
#define ll long long
#define double long double
using namespace std;

int h, r, a, w;

inline double calc(double t) {
    return ((w - a) * t * t + h * h * a) / 2 / ((w - a) * t  + h * a);
}


signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);
    cin >> h >> r >> a >> w;
    double l = 0, r = h;
    for(int i = 1; i <= 200; i++) {
        double lmid = (2 * l + r) / 3, rmid = (l + 2 * r) / 3;
        if(calc(lmid) < calc(rmid)) r = rmid;
        else l = lmid;
    }
    cout << setprecision(20) << l << '\n';
}