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

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();
            double n = x.Sum() - x.Max() - x.Min();
            Console.Write(n / 4);
            if (n % 4 == 0)
            {
                Console.WriteLine(".00");
            }
            else Console.WriteLine(n % 2 == 0 ? "0" : "");
        }
    }
}