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


class Program
{
    public void Proc()
    {
        Reader.IsDebug = false;
        int[] pos1 = Reader.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();
        int[] pos2 = Reader.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();

        decimal kyori = Math.Abs(pos1[0] - pos2[0]) + Math.Abs(pos1[1] - pos2[1]);

        Console.WriteLine(kyori / 2);

    }

    public class Reader
    {
        public static bool IsDebug = true;
        private static String PlainInput = @"





0 0
3 0





";
        private static System.IO.StringReader Sr = null;
        public static string ReadLine()
        {
            if (IsDebug)
            {
                if (Sr == null)
                {
                    Sr = new System.IO.StringReader(PlainInput.Trim());
                }
                return Sr.ReadLine();
            }
            else
            {
                return Console.ReadLine();
            }
        }
    }
    static void Main()
    {
        Program prg = new Program();
        prg.Proc();
    }
}