using System;
using System.Linq;

class Program
{
    public void Solve()
    {
        int N = int.Parse(Console.ReadLine());
        int[] A = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();

        int[] kadomatsu = new int[3];
        int kadomatsuCnt = 0;
        bool isRandom = true;
        for (int i = 0; i < N - 2; i++)
        {
            isRandom = true;
            kadomatsu = new int[3];

            for (int j = 0; j < 3; j++)
            {
                kadomatsu[j] = A[i + j];

                if (kadomatsu.Count(x => x == kadomatsu[j]) > 1)
                {
                    isRandom = !isRandom;
                    break;
                }
            }
            if (!isRandom) { continue; }

            Array.Sort(kadomatsu);

            if (kadomatsu[kadomatsu.Length - 2] == A[i + 1]) { continue; }

            kadomatsuCnt++;
        }

        Console.WriteLine(kadomatsuCnt);
    }

    static void Main()
    {
        var solver = new Program();
        solver.Solve();
    }
}