using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static double[] NList => ReadLine().Split().Select(double.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (x, y, z, w) = (c[0], c[1], c[2], c[3]);
        var ok = z;
        var ng = x / y;
        while (ok - ng > 0.0000001)
        {
            var mid = (ok + ng) / 2;
            var bp = Math.Sqrt(mid * mid + y * y);
            var bq = Math.Sqrt(x / mid * x / mid + z * z);
            if (bp >= bq + w) ok = mid;
            else ng = mid;
        }
        WriteLine(x - ok * y / 2 - x / ok * z / 2 - (ok - z) * (x / ok - y) / 2);
    }
}