import java.util.*;
import java.lang.*;
import java.io.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

        int count = 0;
        
        for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        }
        
        for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        }
        
        System.out.println(count);

    }

} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

        int count = 0;
        
        for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        }
        
        for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        }
        
        System.out.println(count);

    }

} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

        int count = 0;
        
        for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        }
        
        for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        }
        
        System.out.println(count);

    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

        int count = 0;
        
        for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        }
        
        for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        }
        
        System.out.println(count);

    } 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+1]; p new int[n+1] n+1 n 1 int count = 0; count 0 for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        } int i = 1; int i = 1; i 1 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 for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        } int i = 2; int i = 2; i 2 i<n i n i++ i++ i {
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        } {
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        } if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            } 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 {
                count++;
            } count++ count if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            } 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 {
                count++;
            } count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

        int count = 0;
        
        for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        }
        
        for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        }
        
        System.out.println(count);

    }

} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

        int count = 0;
        
        for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        }
        
        for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        }
        
        System.out.println(count);

    }

} Main
import java.util.*;
import java.lang.*;
import java.io.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n+1];

        int count = 0;
        
        for(int i = 1; i <= n; i++){
            p[i] = sc.nextInt();
        }
        
        for(int i = 2; i<n; i++){
            if(p[i] < p[i-1] && p[i] > p[i+1]){
                count++;
            }
            if(p[i] > p[i-1] && p[i] < p[i+1]){
                count++;
            }
        }
        
        System.out.println(count);

    }

}
