
import java.util.Scanner;

public class Main {

	public static void main(String[] args){
		Scanner in = new Scanner(System.in);
		int n = in.nextInt();
		int[] p = new int[n];
		int ans = 0;

		for(int i = 0; i < n; i++)
			p[i] = in.nextInt();

		for(int i = 1; i<n-1; i++) {
			boolean dontMax = (p[i] < Math.max(p[i-1], p[i+1]));
			boolean dontMin = (p[i] > Math.min(p[i-1], p[i+1]));
			if(dontMax && dontMin)
				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