using System;
using System.Linq;

class Program
{
    class Point
    {
        public int X { get; set; }
        public int Y { get; set; }
        public Point(int x, int y)
        {
            X = x;
            Y = y;
        }
    }

    static void Main(string[] args)
    {
        int[] input;
        var points = new Point[] {  new Point(0, 0),
                                    new Point(0, 0) };

        for (int i=0; i < 2; i++)
        {
            input = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
            points[i] = new Point(input[0], input[1]);
        }

        var dx = (Math.Abs(points[0].X - points[1].X)) / 2.0;
        var dy = (Math.Abs(points[0].Y - points[1].Y)) / 2.0;
        var time = dx + dy;
        Console.WriteLine(time);
    }
}