import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = Integer.parseInt(sc.next());

		int[] line = new int[n];
		int count = 0;

		for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		}

		for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		}

		System.out.println(count);

	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = Integer.parseInt(sc.next());

		int[] line = new int[n];
		int count = 0;

		for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		}

		for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		}

		System.out.println(count);

	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = Integer.parseInt(sc.next());

		int[] line = new int[n];
		int count = 0;

		for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		}

		for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		}

		System.out.println(count);

	} main {
		Scanner sc = new Scanner(System.in);

		int n = Integer.parseInt(sc.next());

		int[] line = new int[n];
		int count = 0;

		for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		}

		for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		}

		System.out.println(count);

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int[] line = new int[n]; line new int[n] n int count = 0; count 0 for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			line[i] = Integer.parseInt(sc.next());
		} {
			line[i] = Integer.parseInt(sc.next());
		} line[i] = Integer.parseInt(sc.next()) line[i] line i Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		} int i = 1; int i = 1; i 1 i < n - 1 i n - 1 n 1 i++ i++ i {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		} {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		} if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			} (line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1]) (line[i] > line[i - 1] && line[i] < line[i + 1]) line[i] > line[i - 1] line[i] line i line[i - 1] line i - 1 i 1 line[i] < line[i + 1] line[i] line i line[i + 1] line i + 1 i 1 (line[i] < line[i - 1] && line[i] > line[i + 1]) line[i] < line[i - 1] line[i] line i line[i - 1] line i - 1 i 1 line[i] > line[i + 1] line[i] line i line[i + 1] line i + 1 i 1 {
				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 = Integer.parseInt(sc.next());

		int[] line = new int[n];
		int count = 0;

		for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		}

		for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		}

		System.out.println(count);

	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = Integer.parseInt(sc.next());

		int[] line = new int[n];
		int count = 0;

		for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		}

		for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		}

		System.out.println(count);

	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = Integer.parseInt(sc.next());

		int[] line = new int[n];
		int count = 0;

		for (int i = 0; i < n; i++) {
			line[i] = Integer.parseInt(sc.next());
		}

		for (int i = 1; i < n - 1; i++) {
			if ((line[i] > line[i - 1] && line[i] < line[i + 1])
 			 || (line[i] < line[i - 1] && line[i] > line[i + 1])) {
				count++;
			}
		}

		System.out.println(count);

	}
}