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());
        }
        sc.close();
        int res = 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])){
                res++;
            }
        }
        System.out.println(res);
    }
} 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());
        }
        sc.close();
        int res = 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])){
                res++;
            }
        }
        System.out.println(res);
    }
} 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());
        }
        sc.close();
        int res = 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])){
                res++;
            }
        }
        System.out.println(res);
    } 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());
        }
        sc.close();
        int res = 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])){
                res++;
            }
        }
        System.out.println(res);
    } 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 sc.close() sc.close sc int res = 0; res 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])){
                res++;
            }
        } 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])){
                res++;
            }
        } {
            if((p[i] > p[i-1] && p[i] < p[i+1]) || (p[i] > p[i+1] && p[i] < p[i-1])){
                res++;
            }
        } if((p[i] > p[i-1] && p[i] < p[i+1]) || (p[i] > p[i+1] && p[i] < p[i-1])){
                res++;
            } (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 {
                res++;
            } res++ res System.out.println(res) System.out.println System.out System System.out res 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());
        }
        sc.close();
        int res = 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])){
                res++;
            }
        }
        System.out.println(res);
    }
} 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());
        }
        sc.close();
        int res = 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])){
                res++;
            }
        }
        System.out.println(res);
    }
} 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());
        }
        sc.close();
        int res = 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])){
                res++;
            }
        }
        System.out.println(res);
    }
}
