import java.util.Scanner;

public class Main {
	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int[] p = new int[n];
		
		for (int i = 0; i < n; i++) {
			p[i] = scan.nextInt();
		}
		
		int[] three = new int[3];
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			three[0] = p[i - 1];
			three[1] = p[i];
			three[2] = p[i + 1];
			
			if (three[1] >= three[0] && three[1] < three[2]) count++;
			if(three[1] < three[0] && three[1] >= three[2]) 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