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();
    }
    
    int ans = 0;
    for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} 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();
    }
    
    int ans = 0;
    for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} 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();
    }
    
    int ans = 0;
    for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    }
    System.out.println(ans);
  } 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();
    }
    
    int ans = 0;
    for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    }
    System.out.println(ans);
  } 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 int ans = 0; ans 0 for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    } int i = 2; int i = 2; i 2 i < n i n i++ i++ i {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    } {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    } int p1 = p[i-2]; p1 p[i-2] p i-2 i 2 int p2 = p[i-1]; p2 p[i-1] p i-1 i 1 int p3 = p[i]; p3 p[i] p i if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      } p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1 p1 < p2 && p2 < p3 p1 < p2 p1 p2 p2 < p3 p2 p3 p3 < p2 && p2 < p1 p3 < p2 p3 p2 p2 < p1 p2 p1 {
        ans++;
      } ans++ ans System.out.println(ans) System.out.println System.out System System.out ans 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();
    }
    
    int ans = 0;
    for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} 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();
    }
    
    int ans = 0;
    for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} 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();
    }
    
    int ans = 0;
    for (int i = 2; i < n; i++) {
      int p1 = p[i-2];
      int p2 = p[i-1];
      int p3 = p[i];
      if (p1 < p2 && p2 < p3 || p3 < p2 && p2 < p1) {
        ans++;
      }
    }
    System.out.println(ans);
  }
}