import java.util.Scanner;

public class Run {
    public static void main (String arg[]) {
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        int tmp2 = scan.nextInt();
        int tmp1 = scan.nextInt();
        int A, count = 0;
        for (int i = 0; i < N - 2; i++) {
            A = scan.nextInt();
            if (tmp2 != A) {
                if (tmp2 < tmp1) {
                    if (tmp1 > A) count++;
                } else if (tmp2 > tmp1) {
                    if (tmp1 < A) count++;
                }
            }
            tmp2 = tmp1;
            tmp1 = A;
        }
        System.out.println(count);
    }
}