import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      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