#include <bits/stdc++.h>

using namespace std;

long double h, r, da, dw;

long double calc(long double a){
    long double up = a * a * dw + (h * h - a * a) * da;
    long double down = a * dw + (h - a) * da;
    return up / down;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);

    
    cin >> h >> r >> da >> dw;

    long double l2, r2;
    l2 = 0, r2 = h;

    long double EPS = 1e-9;
    while(r2 - l2 > EPS){
        long double mid1 = (2 * l2 + r2) / 3;
        long double mid2 = (l2 + 2 * r2) / 3;
        if(calc(mid1) < calc(mid2))
            r2 = mid2;
        else
            l2 = mid1;
    }

    cout << fixed << setprecision(9) << l2 << "\n";
}