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[] S = new int[n];
		for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		}
		sc.close();
		int count = 0;
		for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		}
		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[] S = new int[n];
		for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		}
		sc.close();
		int count = 0;
		for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		}
		System.out.println(count);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		int[] S = new int[n];
		for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		}
		sc.close();
		int count = 0;
		for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		}
		System.out.println(count);
	} main {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		int[] S = new int[n];
		for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		}
		sc.close();
		int count = 0;
		for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		}
		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[] S = new int[n]; S new int[n] n for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			S[i] = sc.nextInt();
		} {
			S[i] = sc.nextInt();
		} S[i] = sc.nextInt() S[i] S i sc.nextInt() sc.nextInt sc sc.close() sc.close sc int count = 0; count 0 for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		} int i = 1; int i = 1; i 1 i <= n - 2 i n - 2 n 2 i++ i++ i {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		} {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		} if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			} (S[i + 1] > S[i]) S[i + 1] S i + 1 i 1 S[i] S i {
				if(S[i] > S[i - 1]) {
					count++;
				}
			} if(S[i] > S[i - 1]) {
					count++;
				} S[i] > S[i - 1] S[i] S i S[i - 1] S i - 1 i 1 {
					count++;
				} count++ count if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			} (S[i + 1] < S[i]) S[i + 1] S i + 1 i 1 S[i] S i {
				if(S[i] < S[i - 1]) {
					count++;
				}
			} if(S[i] < S[i - 1]) {
					count++;
				} S[i] < S[i - 1] S[i] S i S[i - 1] S i - 1 i 1 {
					count++;
				} count++ count {
				continue;
			} continue; 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[] S = new int[n];
		for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		}
		sc.close();
		int count = 0;
		for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		}
		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[] S = new int[n];
		for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		}
		sc.close();
		int count = 0;
		for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		}
		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[] S = new int[n];
		for(int i = 0; i < n; i++) {
			S[i] = sc.nextInt();
		}
		sc.close();
		int count = 0;
		for(int i = 1; i <= n - 2; i++) {
			if((S[i + 1] > S[i])){
				if(S[i] > S[i - 1]) {
					count++;
				}
			}else if((S[i + 1] < S[i])){
				if(S[i] < S[i - 1]) {
					count++;
				}
			}else {
				continue;
			}
		}
		System.out.println(count);
	}
}