using System;
using System.Linq;

namespace Yukicoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var A = Console.ReadLine().Split(' ').Select(x=>double.Parse(x)).ToArray();
            var B = Console.ReadLine().Split(' ').Select(x => double.Parse(x)).ToArray();

            double difX = Math.Abs(A[0] - B[0]);
            double difY = Math.Abs(A[1] - B[1]);

            Console.WriteLine((difX + difY) / 2);
        }
    }
}