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

class Program
{
    public void Proc()
    {
        Reader.IsDebug = false;
        int itemCount = int.Parse(Reader.ReadLine());
        int[] items = Reader.GetInt();
        int ans = 0;
        for (int i = 2; i < items.Length; i++)
        {
            if(items[i-2] == items[i-1] || items[i-1] == items[i] || items[i-2] == items[i]) {
                continue;
            }
            if (items[i - 2] < items[i - 1] && items[i - 1] < items[i])
            {
                continue;
            }
            if (items[i - 2] > items[i - 1] && items[i - 1] > items[i])
            {
                continue;
            }
            ans++;
        }
        Console.WriteLine(ans);


    }






    public class Reader
    {
        public static bool IsDebug = true;
        private static String PlainInput = @"

5
1 4 1 5 2


 
";
        private static System.IO.StringReader Sr = null;
        public static string ReadLine()
        {
            if (IsDebug)
            {
                if (Sr == null)
                {
                    Sr = new System.IO.StringReader(PlainInput.Trim());
                }
                return Sr.ReadLine();
            }
            else
            {
                return Console.ReadLine();
            }
        }
        public static int[] GetInt(char delimiter = ' ', bool trim = false)
        {
            string inptStr = ReadLine();
            if (trim)
            {
                inptStr = inptStr.Trim();
            }
            string[] inpt = inptStr.Split(delimiter);
            int[] ret = new int[inpt.Length];
            for (int i = 0; i < inpt.Length; i++)
            {
                ret[i] = int.Parse(inpt[i]);
            }
            return ret;
        }
    }
    static void Main()
    {
        Program prg = new Program();
        prg.Proc();
    }
}