import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());

        int[] p = new int[n];
        for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int cnt = 0;
        for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());

        int[] p = new int[n];
        for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int cnt = 0;
        for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
} Main public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());

        int[] p = new int[n];
        for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int cnt = 0;
        for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    } main {

        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());

        int[] p = new int[n];
        for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int cnt = 0;
        for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int[] p = new int[n]; p new int[n] n for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            p[i] = Integer.parseInt(sc.next());
        } {
            p[i] = Integer.parseInt(sc.next());
        } p[i] = Integer.parseInt(sc.next()) p[i] p i Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int cnt = 0; cnt 0 for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        } int i = 1; int i = 1; i 1 i < n-1 i n-1 n 1 i++ i++ i {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        } {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        } if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            } (p[i] - p[i-1])*(p[i] - p[i+1]) < 0 (p[i] - p[i-1])*(p[i] - p[i+1]) (p[i] - p[i-1]) p[i] p i p[i-1] p i-1 i 1 (p[i] - p[i+1]) p[i] p i p[i+1] p i+1 i 1 0 {
                cnt++;
            } cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());

        int[] p = new int[n];
        for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int cnt = 0;
        for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
} public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());

        int[] p = new int[n];
        for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int cnt = 0;
        for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());

        int[] p = new int[n];
        for (int i = 0; i < n; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int cnt = 0;
        for (int i = 1; i < n-1 ; i++) {
            if ((p[i] - p[i-1])*(p[i] - p[i+1]) < 0) {
                cnt++;
            }
        }
        System.out.println(cnt);
    }
}