import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt(), cnt = 0, a, b, c;
		a = sc.nextInt();
		b = sc.nextInt();
		for (int i = 2; i < n; i++) {
			c = sc.nextInt();
			if ((a - b) * (b - c) * (c - a) != 0 && (a - b) * (c - b) > 0) {
				cnt++;
			}
			a = b;
			b = c;
		}
		System.out.println(cnt);
	}
}