import java.util.*;

class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    List<Integer>p = new ArrayList<>();
    for(int i=0;i<n;i++){
      p.add(sc.nextInt()-1);
    }
    int ans = 0;
    for(int i=1;i<n-1;i++){
      if((p.get(i)-p.get(i-1))*(p.get(i+1)-p.get(i))>0)ans++;
    }
    System.out.println(ans);

  }
}

/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