using System;

class Program
{
    static void Main()
    {
        string[] p = Console.ReadLine()!.Split();
        string[] q = Console.ReadLine()!.Split();

        long x1 = long.Parse(p[0]);
        long y1 = long.Parse(p[1]);
        long x2 = long.Parse(q[0]);
        long y2 = long.Parse(q[1]);

        double time = (Math.Abs(x2 - x1) + Math.Abs(y2 - y1)) / 2.0;
        Console.WriteLine(time);
    }
}