#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
const int N = 1e4 + 5;
#define int long long
typedef pair<int, int> PII;
#define MAX LONG_LONG_MAX
#define mod 10007
#define eps 1e-7
double h, rr, p1, p2;

double calc(double l)
{
    // double ha = h - l;
    // double mw = l * p2;
    // double ma = ha * p1;
    // double cw = l / 2;
    // double ca = h - ha / 2;
    // return (ca * ma + cw * mw);
    return (l * l * p1 + (h - l) * p2 * (h - l)) / 2 / (l * p1 + (h - l) * p2);
}

signed main()
{

    cin >> h >> rr >> p1 >> p2;

    double l = 0, r = h;

    while (r - l > eps)
    {
        double mid1 = (l + r) / 2;
        double mid2 = (r + mid1) / 2;
        double x = calc(mid1);
        double y = calc(mid2);

        if (x > y)
            l = mid1;
        else
            r = mid2;
    }
    cout << printf("%.10lf", h - l) << endl;

    return 0;
}