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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				count++;
			}
		} int i=0; int i=0; i 0 i<n-2 i n-2 n 2 i++ i++ i {
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				count++;
			}
		} {
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				count++;
			}
		} if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				count++;
			} p[i]>p[i+1] p[i] p i p[i+1] p i+1 i 1 {
				if(p[i+2]<p[i+1]){
					count++;
				}
			} if(p[i+2]<p[i+1]){
					count++;
				} p[i+2]<p[i+1] p[i+2] p i+2 i 2 p[i+1] p i+1 i 1 {
					count++;
				} count++ count if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				count++;
			} p[i]<p[i+1] p[i] p i p[i+1] p i+1 i 1 {
				if(p[i+2]>p[i+1]){
					count++;
				}
			} if(p[i+2]>p[i+1]){
					count++;
				} p[i+2]>p[i+1] p[i+2] p i+2 i 2 p[i+1] p i+1 i 1 {
					count++;
				} count++ count {
				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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				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=0; i<n-2; i++){
			if(p[i]>p[i+1]){
				if(p[i+2]<p[i+1]){
					count++;
				}
			}else if(p[i]<p[i+1]){
				if(p[i+2]>p[i+1]){
					count++;
				}
			}else{
				count++;
			}
		}
		System.out.println(count);
	}
}


