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

class Program
{
    public void Solve()
    {
        int[] P = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
        int[] Q = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();

        Console.WriteLine((decimal)(Math.Abs(P[0] - Q[0]) + Math.Abs(P[1] - Q[1])) / 2);
    }

    static void Main()
    {
        var solver = new Program();
        solver.Solve();
    }
}