import java.util.*;
public class Main {

  public static void main (String[] args) {

    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] p = new int[n];

    for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    }
    sc.close();

    int count = 0;
    
    for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        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[] p = new int[n];

    for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    }
    sc.close();

    int count = 0;
    
    for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        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];

    for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    }
    sc.close();

    int count = 0;
    
    for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        count++;
      }
    }

    System.out.println(count);

  } main {

    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] p = new int[n];

    for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    }
    sc.close();

    int count = 0;
    
    for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        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]; p new int[n] n for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    } int i = 0; int i = 0; i 0 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 sc.close() sc.close sc int count = 0; count 0 for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        count++;
      }
    } int i=1; int i=1; i 1 i<n-1 i n-1 n 1 i++ i++ i {
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        count++;
      }
    } {
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        count++;
      }
    } int p1 = p[i-1]; p1 p[i-1] p i-1 i 1 int p2 = p[i]; p2 p[i] p i int p3 = p[i+1]; p3 p[i+1] p i+1 i 1 if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        count++;
      } (p1 < p2) && (p2 < p3) (p1 < p2) p1 p2 (p2 < p3) p2 p3 {
        count++;
      } count++ count if ((p3 < p2) && (p2 < p1)) {
        count++;
      } (p3 < p2) && (p2 < p1) (p3 < p2) p3 p2 (p2 < p1) p2 p1 {
        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];

    for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    }
    sc.close();

    int count = 0;
    
    for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        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];

    for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    }
    sc.close();

    int count = 0;
    
    for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        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[] p = new int[n];

    for (int i = 0; i < n; i++) {
      p[i] = sc.nextInt();
    }
    sc.close();

    int count = 0;
    
    for(int i=1; i<n-1; i++){
      int p1 = p[i-1];
      int p2 = p[i];
      int p3 = p[i+1];
      if ((p1 < p2) && (p2 < p3)){
        count++;
      } else if ((p3 < p2) && (p2 < p1)) {
        count++;
      }
    }

    System.out.println(count);

  }
  
}