using System;

public class Hello
{
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var x1 = double.Parse(line[0]);
        var y1 = double.Parse(line[1]);
        line = Console.ReadLine().Trim().Split(' ');
        var x2 = double.Parse(line[0]);
        var y2 = double.Parse(line[1]);
        Console.WriteLine(x1 * (y2 - y1) / (x1 + x2) + y1);
    }
}