package yukicoder116;
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		int[] a=new int[n];
		for(int i=0;i<n;i++){
			a[i]=sc.nextInt();
		}
		int count=0;
		for(int i=0;i<n-2;i++){
			if(a[i]!=a[i+1]&&a[i+1]!=a[i+2]&&a[i]!=a[i+2]){
				if((a[i+1]>a[i]&&a[i+1]>a[i+2])||(a[i+1]<a[i]&&a[i+1]<a[i+2]))
				count++;
			}
		}
		System.out.println(count);
	}
}