import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] l = new int[n];
		for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		}
		int count = 0;
		for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		}
System.out.println(count);
	}
} import java.util.Arrays; Arrays java.util import java.util.HashMap; HashMap java.util import java.util.Map; Map java.util 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[] l = new int[n];
		for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		}
		int count = 0;
		for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		}
System.out.println(count);
	}
} Main public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] l = new int[n];
		for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		}
		int count = 0;
		for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		}
System.out.println(count);
	} main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] l = new int[n];
		for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		}
		int count = 0;
		for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				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[] l = new int[n]; l new int[n] n for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		} int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
			l[i] = sc.nextInt();
		} {
			l[i] = sc.nextInt();
		} l[i] = sc.nextInt() l[i] l i sc.nextInt() sc.nextInt sc int count = 0; count 0 for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		} int i = 0; int i = 0; i 0 i < n - 2 i n - 2 n 2 i++ i++ i {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		} {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		} if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			} (l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i]) (l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) l[i + 1] >= l[i] l[i + 1] l i + 1 i 1 l[i] l i l[i + 1] <= l[i + 2] l[i + 1] l i + 1 i 1 l[i + 2] l i + 2 i 2 (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i]) l[i + 1] >= l[i + 2] l[i + 1] l i + 1 i 1 l[i + 2] l i + 2 i 2 l[i + 1] <= l[i] l[i + 1] l i + 1 i 1 l[i] l i {
				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[] l = new int[n];
		for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		}
		int count = 0;
		for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				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[] l = new int[n];
		for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		}
		int count = 0;
		for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		}
System.out.println(count);
	}
} Main
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] l = new int[n];
		for (int i = 0; i < n; i++) {
			l[i] = sc.nextInt();
		}
		int count = 0;
		for (int i = 0; i < n - 2; i++) {
			if ((l[i + 1] >= l[i] && l[i + 1] <= l[i + 2]) || (l[i + 1] >= l[i + 2] && l[i + 1] <= l[i])) {
				count++;
			}
			
		}
System.out.println(count);
	}
}
