import java.util.*;

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