import java.util.*;
import java.io.*;



public class Main{
    static int INFTY=Integer.MAX_VALUE;
    public static void main (String args[]){
        //初期処理
        Scanner sc=new Scanner(System.in);
        // BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        // try{
            // int n = Integer.parseInt(br.readLine());
            
        int n=sc.nextInt();
        int[] arr = new int[n];
        int ans=0;

        for(int i=0;i<n;i++){
            arr[i]=sc.nextInt();
        }
        for(int i=0;i<n-2;i++){
            if((arr[i]<arr[i+1]&&arr[i+1]<arr[i+2])||(arr[i+2]<arr[i+1]&&arr[i+1]<arr[i])){
             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