import java.util.Scanner;
public class No116 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		int count = 0;
		for (int i = 0; i < n; i++)
			a[i] = sc.nextInt();
		for (int i = 0; i < n - 2; i++)
			if ((a[i] < a[i + 1] && a[i + 1] > a[i + 2]&&a[i]!=a[i+2])||(a[i] > a[i + 1] && a[i + 1] < a[i + 2]&&a[i]!=a[i+2]))
				count++;
		System.out.println(count);
	}
}