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