import java.util.*;

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

public class Main{
    public static void main(String[] args){
        Scanner scan = new java.util.Scanner(System.in);
        int n = scan.nextInt();
        int[] p = new int[n];
        int a = 0;
        for(int i = 0; i < n; i++){
            p[i] = scan.nextInt();
        }
        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]){
                a++;
            }
        }
        System.out.println(a);
    }
}