import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        Integer[] p = new Integer[n];
        for (int i = 0; i < n; i++) {
            p[i] = sc.nextInt();
        }
        sc.close();

        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (p[i - 1] == p[i + 1]) {
                if (p[i] < p[i + 1]) {
                    count++;
                }
            } else if (p[i - 1] < p[i + 1]) {
                if (p[i - 1] <= p[i] && p[i] < p[i + 1]) {
                    count++;
                }
            } else if (p[i - 1] > p[i + 1]) {
                if (p[i - 1] > p[i] && p[i] >= p[i + 1]) {
                    count++;
                }
            }
        }

        System.out.println(count);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz