using System;

class Program
{
    static void Main()
    {
     var a  =  scan();
     var b  =  scan();
     double c = Math.Abs(a[0]-b[0])+Math.Abs(a[1]-b[1]);
     c/=2;
     Console.WriteLine(c);
    }
    static double[] scan()
    {
        return Array.ConvertAll(Console.ReadLine().Split(' '),double.Parse);
    }
}