import java.util.Scanner;

public class Main{

	public static void main(String[] args) {

		Scanner scanner = new Scanner(System.in);

		int n = Integer.parseInt(scanner.nextLine());
		String[] temp = scanner.nextLine().split(" ");
		int[] list = new int[n];
		int ans   = 0;



		for(int i = 0; i < n; i++) {


			list[i] = Integer.parseInt(temp[i]);

			if(i > 1) {

				if((list[i - 2] > list[i - 1] && list[i] < list[i - 1]) ||(list[i - 2] < list[i - 1] && list[i] > list[i - 1])) {
				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