using System;

public class Hello
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var px = int.Parse(line[0]);
        var py = int.Parse(line[1]);
        line = Console.ReadLine().Trim().Split(' ');
        var qx = int.Parse(line[0]);
        var qy = int.Parse(line[1]);
        var mtn = Math.Abs(px - qx) + Math.Abs(py - qy);
        Console.WriteLine((mtn / 2d));
    }
}