import java.io.FileNotFoundException;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		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 ans = 0;
		for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		}
		System.out.println(ans);
	}
} import java.io.FileNotFoundException; FileNotFoundException java.io import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		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 ans = 0;
		for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		}
		System.out.println(ans);
	}
} Main public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		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 ans = 0;
		for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		}
		System.out.println(ans);
	} main {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		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 ans = 0;
		for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		}
		System.out.println(ans);
	} 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 ans = 0; ans 0 for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		} 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])>0){
				ans ++;
			}
		} {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		} if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			} (p[i-1]-p[i])*(p[i]-p[i+1])>0 (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 0 {
				ans ++;
			} ans ++ ans System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
	public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		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 ans = 0;
		for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		}
		System.out.println(ans);
	}
} public class Main {
	public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		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 ans = 0;
		for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		}
		System.out.println(ans);
	}
} Main
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		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 ans = 0;
		for(int i=1;i<n-1;i++) {
			if((p[i-1]-p[i])*(p[i]-p[i+1])>0){
				ans ++;
			}
		}
		System.out.println(ans);
	}
}