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

class Program
{
    static void Main(string[] args)
    {
        int[] a = Console.ReadLine().Split().Select(v => int.Parse(v)).ToArray();
        int[] b = Console.ReadLine().Split().Select(v => int.Parse(v)).ToArray();

        double d = (Math.Abs(a[0] - b[0]) + Math.Abs(a[1] - b[1])) / 2.0;

        Console.WriteLine(d);
        Console.Read();
    }
}