using System;
using System.Collections.Generic;
using System.Linq;

//YukiCoder410 出会い
//http://yukicoder.me/problems/no/410
class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("0 0");
            WillReturn.Add("2 0");
            //1
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("0 0");
            WillReturn.Add("3 0");
            //1.5
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("0 0");
            WillReturn.Add("5 1");
            //3
        }
        else if (InputPattern == "Input4") {
            WillReturn.Add("-12345 6789");
            WillReturn.Add("1234 -56789");
            //38578.5
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();

        decimal[] wkArr = { };
        Action<string> SplitAct = pStr =>
            wkArr = pStr.Split(' ').Select(X => decimal.Parse(X)).ToArray();

        SplitAct(InputList[0]);
        decimal PX = wkArr[0], PY = wkArr[1];

        SplitAct(InputList[1]);
        decimal QX = wkArr[0], QY = wkArr[1];

        Console.WriteLine(Math.Abs(PX - QX) / 2M + Math.Abs(PY - QY) / 2M);
    }
}