using System;
using System.Collections.Generic;
using System.Linq;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            double[] x = Console.ReadLine().Split().Select(double.Parse).ToArray();
            double[] y = Console.ReadLine().Split().Select(double.Parse).ToArray();
            Console.WriteLine((Math.Abs(x[0] - y[0]) + Math.Abs(x[1] - y[1])) / 2);
        }
    }
}