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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        count++;
      }
    } int i = 0; int i = 0; i 0 i < n - 2 i n - 2 n 2 i++ i++ i {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        count++;
      }
    } {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        count++;
      }
    } int a = list[i]; a list[i] list i int b = list[i+1]; b list[i+1] list i+1 i 1 int c = list[i+2]; c list[i+2] list i+2 i 2 if((b > a && b < c) || (b < a && b > c)) {
        count++;
      } (b > a && b < c) || (b < a && b > c) (b > a && b < c) b > a b a b < c b c (b < a && b > c) b < a b a b > c b c {
        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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        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++) {
      int a = list[i];
      int b = list[i+1];
      int c = list[i+2];

      if((b > a && b < c) || (b < a && b > c)) {
        count++;
      }
    }

    System.out.println(count);
  }
}