import java.util.Scanner;

public class Main {

	public static void main (String[] args) {

		Scanner sc = new Scanner (System.in);
		
		int N = sc.nextInt();
		
		int[] list = new int [N];
		
		for (int index=0; index<N; index++){
		    list[index]=sc.nextInt();
		}

        int count = 0;
        
		for (int index=1; index<N-1; index++){
		    if (list[index]<list[index+1]&&list[index]>list[index-1]){
		        count++;
		    } else if (list[index]>list[index+1]&&list[index]<list[index-1]){
		        count ++;
		    }
		}
		
		System.out.println(count);

		sc.close();
	}
}
/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