import java.util.*;

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