import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    ArrayList<Integer> lists = new ArrayList<Integer>();
    for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    }
    int cnt = 0;
    for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    }
    System.out.println(cnt);
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    ArrayList<Integer> lists = new ArrayList<Integer>();
    for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    }
    int cnt = 0;
    for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    }
    System.out.println(cnt);
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    ArrayList<Integer> lists = new ArrayList<Integer>();
    for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    }
    int cnt = 0;
    for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    }
    System.out.println(cnt);
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    ArrayList<Integer> lists = new ArrayList<Integer>();
    for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    }
    int cnt = 0;
    for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    }
    System.out.println(cnt);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc ArrayList<Integer> lists = new ArrayList<Integer>(); lists new ArrayList<Integer>() for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
      lists.add(sc.nextInt());
    } {
      lists.add(sc.nextInt());
    } lists.add(sc.nextInt()) lists.add lists sc.nextInt() sc.nextInt sc int cnt = 0; cnt 0 for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    } int i = 1; int i = 1; i 1 i < N - 1 i N - 1 N 1 i++ i++ i {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    } {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    } if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      } (lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1)) (lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1)) lists.get(i) > lists.get(i - 1) lists.get(i) lists.get lists i lists.get(i - 1) lists.get lists i - 1 i 1 lists.get(i) < lists.get(i + 1) lists.get(i) lists.get lists i lists.get(i + 1) lists.get lists i + 1 i 1 (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1)) lists.get(i) < lists.get(i - 1) lists.get(i) lists.get lists i lists.get(i - 1) lists.get lists i - 1 i 1 lists.get(i) > lists.get(i + 1) lists.get(i) lists.get lists i lists.get(i + 1) lists.get lists i + 1 i 1 {
        cnt++;
      } cnt++ cnt System.out.println(cnt) System.out.println System.out System System.out cnt String[] args args public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    ArrayList<Integer> lists = new ArrayList<Integer>();
    for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    }
    int cnt = 0;
    for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    }
    System.out.println(cnt);
  }
} public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    ArrayList<Integer> lists = new ArrayList<Integer>();
    for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    }
    int cnt = 0;
    for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    }
    System.out.println(cnt);
  }
} Main
import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    ArrayList<Integer> lists = new ArrayList<Integer>();
    for (int i = 0; i < N; i++) {
      lists.add(sc.nextInt());
    }
    int cnt = 0;
    for (int i = 1; i < N - 1; i++) {
      if ((lists.get(i) > lists.get(i - 1) && lists.get(i) < lists.get(i + 1))
          || (lists.get(i) < lists.get(i - 1) && lists.get(i) > lists.get(i + 1))) {
        cnt++;
      }
    }
    System.out.println(cnt);
  }
}
