#include <bits/stdc++.h>

#define double long double

using namespace std;

double h, R, da, dw;
const double eps = 1e-9;

double calc(double x) {
    double fz = da * (h * h - x * x) + dw * x * x;
    double fm = da * (h - x) + dw * x;
    return fz / fm;
}

int main() {
    cin >> h >> R >> da >> dw;
    double l = 0, r = h;
    while(r - l > eps) {
        double p1 = l + (r - l) / 3;
        double p2 = l + (r - l) / 3 * 2;
        double a1 = calc(p1), a2 = calc(p2);
        if(a1 < a2) r = p2;
        else l = p1;
    }
    // printf("%.9lf\n", (l + r) / 2);
    cout << fixed << setprecision(10);
    cout << (l + r) / 2 << endl;
    return 0;
}