import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        for (int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        int count = 0;
        for (int i = 0; i < n - 2; i++) {
            if (((arr[i] < arr[i + 1] && arr[i + 1] > arr[i + 2]) || (arr[i] > arr[i + 1] && arr[i + 1] < arr[i + 2])) && arr[i] != arr[i + 2]) {
                count++;
            }
        }
        System.out.println(count);
    }
}