using System;
using System.Linq;

public class P0116 {
    public static void Main() {
        var n = int.Parse(Console.ReadLine().Trim());
        var a = Console.ReadLine().Trim().Split(' ').Select(int.Parse).ToList();
        var result = 0;
        for(int i=0; i<n-2; i++) {
            var cands = new int[a[i], a[i+1], a[i+2]];
            if (a[i+1] < a[i] && a[i+1] < a[i+2] && a[i] != a[i+2]) {
                result += 1;
            } else if(a[i+1] > a[i] && a[i+1] > a[i+2] && a[i] != a[i+2]) {
                result += 1;
            }
        }
        Console.WriteLine(result);
    }
}