import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int[] p = new int[N];
		for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		}
		sc.close();

		int cnt = 0;
		for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		}
		System.out.println(cnt);
	}
} 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.nextLine());
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int[] p = new int[N];
		for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		}
		sc.close();

		int cnt = 0;
		for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		}
		System.out.println(cnt);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int[] p = new int[N];
		for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		}
		sc.close();

		int cnt = 0;
		for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		}
		System.out.println(cnt);
	} main {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int[] p = new int[N];
		for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		}
		sc.close();

		int cnt = 0;
		for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		}
		System.out.println(cnt);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = Integer.parseInt(sc.nextLine()); N Integer.parseInt(sc.nextLine()) Integer.parseInt Integer sc.nextLine() sc.nextLine sc String s = sc.nextLine(); s sc.nextLine() sc.nextLine sc String[] spl = s.split(" "); spl s.split(" ") s.split s " " int[] p = new int[N]; p new int[N] N for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		} int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
			p[i] = Integer.parseInt(spl[i]);
		} {
			p[i] = Integer.parseInt(spl[i]);
		} p[i] = Integer.parseInt(spl[i]) p[i] p i Integer.parseInt(spl[i]) Integer.parseInt Integer spl[i] spl i sc.close() sc.close sc int cnt = 0; cnt 0 for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		} int i = 1; int i = 1; i 1 i < N - 1 i N - 1 N 1 i++ i++ i {
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		} {
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		} if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			} (p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1] (p[i - 1] < p[i] &&  p[i] < p[i + 1]) p[i - 1] < p[i] p[i - 1] p i - 1 i 1 p[i] p i p[i] < p[i + 1] p[i] p i p[i + 1] p i + 1 i 1 p[i - 1] > p[i] &&  p[i] > p[i + 1] p[i - 1] > p[i] p[i - 1] p i - 1 i 1 p[i] p i p[i] > p[i + 1] p[i] p i p[i + 1] p 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 = Integer.parseInt(sc.nextLine());
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int[] p = new int[N];
		for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		}
		sc.close();

		int cnt = 0;
		for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		}
		System.out.println(cnt);
	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = Integer.parseInt(sc.nextLine());
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int[] p = new int[N];
		for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		}
		sc.close();

		int cnt = 0;
		for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		}
		System.out.println(cnt);
	}
} 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.nextLine());
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int[] p = new int[N];
		for(int i = 0;i < N;i++){
			p[i] = Integer.parseInt(spl[i]);
		}
		sc.close();

		int cnt = 0;
		for(int i = 1;i < N - 1;i++){
			if((p[i - 1] < p[i] &&  p[i] < p[i + 1]) ||
					p[i - 1] > p[i] &&  p[i] > p[i + 1]){
				cnt++;
			}
		}
		System.out.println(cnt);
	}
}