#include<bits/stdc++.h>
//#define int long long
#define endl "\n"
#define ll long long
#define PII std::pair<int,int>
using namespace std;
const int maxn = 1e4 + 5;
const double eps = 1e-8;
double r, h, da, dw;
double f(double hw) {
    double ha = h - hw;
    double mw = hw * dw;
    double ma = ha * da;
    double cw = hw / 2;
    double ca = h - ha / 2;
    return (ca * ma + cw * mw) / (ma + mw);
}
int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    cout << fixed << setprecision(12);
    cin >> h >> r >> da >> dw;
    double l = 0, r = h;
    while (fabs(l - r) > eps) {
        double mid1 = (l + r) / 2;
        double mid2 = (r + mid1) / 2;
        if (f(mid1) > f(mid2)) l = mid1;
        else r = mid2;
    }
    cout << l << endl;
    return 0;
}