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