import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] list = new int[n];
        
        for (int i = 0; i < n; i++) {
            list[i]= sc.nextInt();
        }
        int c = 0;
        for (int i = 1; i < n-1; i++) {
            if( (list[i] > list[i-1] && list[i] < list[i+1]) ||
                    (list[i] < list[i-1] && list[i] > list[i+1]) ) {
                c++;
            }
        }
        

        System.out.println(c);   
        }
}

/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