#line 1 "main.cpp"
#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;
using ldouble = long double;

void solve() {
    ldouble x, y, z, d;
    cin >> x >> y >> z >> d;

    ldouble lt = z, gt = x / y;
    for (int q = 0; q < 100; ++q) {
        auto h = (lt + gt) / 2;
        auto w = x / h;

        auto bp = sqrt(h * h + y * y),
             bq = sqrt(w * w + z * z);
        if (bp > bq + d) {
            gt = h;
        } else {
            lt = h;
        }
    }

    auto h = lt;
    auto w = x / h;

    auto ans = x - (h * y + z * w + (h - z) * (w - y)) / 2;
    cout << ans << "\n";
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);

    solve();

    return 0;
}