import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		List<Integer> p = new ArrayList<>();
		for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		}
		int judge = 0;
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				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();
		List<Integer> p = new ArrayList<>();
		for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		}
		int judge = 0;
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				count++;
			}
		}
		System.out.println(count);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		List<Integer> p = new ArrayList<>();
		for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		}
		int judge = 0;
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				count++;
			}
		}
		System.out.println(count);
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		List<Integer> p = new ArrayList<>();
		for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		}
		int judge = 0;
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				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 List<Integer> p = new ArrayList<>(); p new ArrayList<>() for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			p.add(sc.nextInt());
		} {
			p.add(sc.nextInt());
		} p.add(sc.nextInt()) p.add p sc.nextInt() sc.nextInt sc int judge = 0; judge 0 int count = 0; count 0 for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				count++;
			}
		} int i = 1; int i = 1; i 1 i < n - 1 i n - 1 n 1 i++ i++ i {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				count++;
			}
		} {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				count++;
			}
		} judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)) judge (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)) (p.get(i) - p.get(i - 1)) p.get(i) p.get p i p.get(i - 1) p.get p i - 1 i 1 (p.get(i) - p.get(i + 1)) p.get(i) p.get p i p.get(i + 1) p.get p i + 1 i 1 if (judge < 0) {
				count++;
			} judge < 0 judge 0 {
				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();
		List<Integer> p = new ArrayList<>();
		for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		}
		int judge = 0;
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				count++;
			}
		}
		System.out.println(count);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		List<Integer> p = new ArrayList<>();
		for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		}
		int judge = 0;
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				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();
		List<Integer> p = new ArrayList<>();
		for (int i = 0; i < n; i++) {
			p.add(sc.nextInt());
		}
		int judge = 0;
		int count = 0;
		for (int i = 1; i < n - 1; i++) {
			judge = (p.get(i) - p.get(i - 1)) * (p.get(i) - p.get(i + 1)); 
			if (judge < 0) {
				count++;
			}
		}
		System.out.println(count);
	}
}