import java.util.*;
public class Main {

  public static void main (String[] args) {

    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] p = new int[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++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        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