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

namespace yukicoderTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] input1 = Console.ReadLine().Split(' ');
            string[] input2 = Console.ReadLine().Split(' ');
            int ax = int.Parse(input1[0]);
            int ay = int.Parse(input1[1]);
            int bx = int.Parse(input2[0]);
            int by = int.Parse(input2[1]);
            double x = 0;
            double y = 0;
            if (ax > bx)
            {
                x = ax - bx;
            }
            else
            {
                x = bx - ax;
            }
            if (ay > by)
            {
                y = ay - by;
            }
            else
            {
                y = by - ay;
            }
            double ans = (x + y) / 2;
            Console.WriteLine(ans);
            }

    }
}