import java.util.Scanner;

public class Main{

    public static void main(String args[]){

        Scanner scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer p[] = new Integer[n];

        for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next());

        int cnt = 0;
        for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        }

        System.out.println(cnt);

    }
} import java.util.Scanner; Scanner java.util public class Main{

    public static void main(String args[]){

        Scanner scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer p[] = new Integer[n];

        for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next());

        int cnt = 0;
        for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        }

        System.out.println(cnt);

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

        Scanner scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer p[] = new Integer[n];

        for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next());

        int cnt = 0;
        for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        }

        System.out.println(cnt);

    } main {

        Scanner scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer p[] = new Integer[n];

        for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next());

        int cnt = 0;
        for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        }

        System.out.println(cnt);

    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) Integer n = Integer.parseInt(scan.next()); n Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan Integer p[] = new Integer[n]; p new Integer[n] n for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next()); int i=0; int i=0; i 0 i<n i n i++ i++ i p[i] = Integer.parseInt(scan.next()); p[i] = Integer.parseInt(scan.next()) p[i] p i Integer.parseInt(scan.next()) Integer.parseInt Integer scan.next() scan.next scan int cnt = 0; cnt 0 for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && 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] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        } {
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        } if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            } (p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1]) (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 (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 {
                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 scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer p[] = new Integer[n];

        for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next());

        int cnt = 0;
        for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        }

        System.out.println(cnt);

    }
} public class Main{

    public static void main(String args[]){

        Scanner scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer p[] = new Integer[n];

        for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next());

        int cnt = 0;
        for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        }

        System.out.println(cnt);

    }
} Main
import java.util.Scanner;

public class Main{

    public static void main(String args[]){

        Scanner scan = new Scanner(System.in);
        Integer n = Integer.parseInt(scan.next());
        Integer p[] = new Integer[n];

        for(int i=0;i<n;i++)   
            p[i] = Integer.parseInt(scan.next());

        int cnt = 0;
        for(int i=1;i<n-1;i++){
            if((p[i] < p[i-1] && p[i] > p[i+1]) || (p[i] > p[i-1] && p[i] < p[i+1])){
                cnt++;
            }
        }

        System.out.println(cnt);

    }
}