using System;

public class Program {
    public static void Main() {
        int n = int.Parse(Console.ReadLine());
        int[] kadomatsu = Array.ConvertAll(Console.ReadLine().Split(), value => int.Parse(value));
        int ans = 0;
        for (int i = 1; i < n-1; i++) {
            if (!((kadomatsu[i-1] < kadomatsu[i] && kadomatsu[i] < kadomatsu[i+1]) || (kadomatsu[i+1] < kadomatsu[i] && kadomatsu[i] < kadomatsu[i-1])) && kadomatsu[i-1] != kadomatsu[i] && kadomatsu[i] != kadomatsu[i+1] && kadomatsu[i+1] != kadomatsu[i-1]) {
                ans++;
            }
        }
        Console.WriteLine(ans);
    }
}