import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		int n = stdIn.nextInt();
		int p[] = new int[n];
		int ans = 0;
		for(int i = 0; i < n; i++){
			p[i] = stdIn.nextInt();
		}
		for(int i = 0; i < n - 2; i++){
			if(p[i] < p[i+1] && p[i+1] < p[i+2] || p[i] > p[i+1] && p[i+1] > p[i+2]){
				ans++;
			}
		}
		System.out.println(ans);
		stdIn.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