using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace yukikoda
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] s = Console.ReadLine().Split();
            double px = int.Parse(s[0]);
            double py = int.Parse(s[1]);
            s = Console.ReadLine().Split();
            double qx = int.Parse(s[0]);
            double qy = int.Parse(s[1]);
            double ans = (Math.Abs(px - qx) + Math.Abs(py - qy)) / 2;
            Console.WriteLine(ans);
        }
    }
}