using System;

namespace yukicoder
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] s = Console.ReadLine().Split(' ');
            string[] s1 = Console.ReadLine().Split(' ');
            int x1 = int.Parse(s[0]);
            int x2 = int.Parse(s1[0]);
            int y1 = int.Parse(s[1]);
            int y2 = int.Parse(s1[1]);
            double ansx = 0;
            double ansy = 0;
            double ans = 0;

            ansx = Math.Abs(x1 - x2);
            ansx /= 2; 
            ansy = Math.Abs(y1 - y2);
            ansy /= 2;
            ans = ansx + ansy;
            Console.WriteLine(ans);
        }
    }
}