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

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

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