import java.util.Scanner;


public class Main_yukicoder116 {

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

        int n = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i++) {
        	a[i] = sc.nextInt();
        }

        int ret = 0;
        for (int i = 1; i <= n - 2; i++) {
        	if (isKadomatu(a, i)) {
        		ret++;
        	}
        }
        
        System.out.println(ret);
        
        sc.close();
    }

	private static boolean isKadomatu(int[] a, int i) {
		if (a[i - 1] == a[i] || a[i] == a[i + 1] || a[i - 1] == a[i + 1]) {
			return false;
		}
		if (a[i] > a[i - 1] && a[i] > a[i + 1]) {
			return true;
		}
		if (a[i] < a[i - 1] && a[i] < a[i + 1]) {
			return true;
		}
		
		return false;
	}
}