import java.util.Scanner;

public class Main {

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

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int p[] = new int[N];
		for (int i=0;i<N;i++) {
			p[i] = sc.nextInt();
		}
		int count =0;
		for (int i=1;i<N-1;i++) {
			if (((p[i]>p[i-1])&&(p[i]<p[i+1]))||(p[i]<p[i-1])&&(p[i]>p[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 = sc.nextInt();
		int p[] = new int[N];
		for (int i=0;i<N;i++) {
			p[i] = sc.nextInt();
		}
		int count =0;
		for (int i=1;i<N-1;i++) {
			if (((p[i]>p[i-1])&&(p[i]<p[i+1]))||(p[i]<p[i-1])&&(p[i]>p[i+1])) {
				count++;
			}
		}
		System.out.println(count);
	}
}
