import java.util.Scanner;

public class Main {

    private static final int MOD = 1_000_000_007;

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

        int n = sc.nextInt();

        int[] p = new int[n];

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

        int cnt = 0;

        for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        }

        System.out.println(cnt);
    }
} import java.util.Scanner; Scanner java.util public class Main {

    private static final int MOD = 1_000_000_007;

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

        int n = sc.nextInt();

        int[] p = new int[n];

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

        int cnt = 0;

        for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        }

        System.out.println(cnt);
    }
} Main private static final int MOD = 1_000_000_007; MOD 1_000_000_007 public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        int[] p = new int[n];

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

        int cnt = 0;

        for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        }

        System.out.println(cnt);
    } main {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();

        int[] p = new int[n];

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

        int cnt = 0;

        for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        }

        System.out.println(cnt);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] p = new int[n]; p new int[n] n for (int i = 0; i < n; i++) {
            p[i] = sc.nextInt();
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            p[i] = sc.nextInt();
        } {
            p[i] = sc.nextInt();
        } p[i] = sc.nextInt() p[i] p i sc.nextInt() sc.nextInt sc int cnt = 0; cnt 0 for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        } int i = 1; int i = 1; i 1 i < n - 1 i n - 1 n 1 i++ i++ i {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        } {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        } if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt; (p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1] (p[i - 1] > p[i] && p[i] > p[i + 1]) p[i - 1] > p[i] p[i - 1] p i - 1 i 1 p[i] p i p[i] > p[i + 1] p[i] p i p[i + 1] p i + 1 i 1 p[i - 1] < p[i] && p[i] < p[i + 1] p[i - 1] < p[i] p[i - 1] p i - 1 i 1 p[i] p i p[i] < p[i + 1] p[i] p i p[i + 1] p i + 1 i 1 ++cnt cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args public class Main {

    private static final int MOD = 1_000_000_007;

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

        int n = sc.nextInt();

        int[] p = new int[n];

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

        int cnt = 0;

        for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        }

        System.out.println(cnt);
    }
} public class Main {

    private static final int MOD = 1_000_000_007;

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

        int n = sc.nextInt();

        int[] p = new int[n];

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

        int cnt = 0;

        for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        }

        System.out.println(cnt);
    }
} Main
import java.util.Scanner;

public class Main {

    private static final int MOD = 1_000_000_007;

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

        int n = sc.nextInt();

        int[] p = new int[n];

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

        int cnt = 0;

        for (int i = 1; i < n - 1; i++) {
            if ((p[i - 1] > p[i] && p[i] > p[i + 1]) || p[i - 1] < p[i] && p[i] < p[i + 1]) ++cnt;
        }

        System.out.println(cnt);
    }
}
