import java.util.Scanner;

class no116{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		int n,i,s=0,a[]=new int[100];
		n=stdIn.nextInt();
		for(i=0;i<n;i++) a[i]=stdIn.nextInt();
		for(i=0;i<n-2;i++) if((a[i]<a[i+1]&&a[i+1]>a[i+2]||a[i]>a[i+1]&&a[i+1]<a[i+2])&&a[i]!=a[i+2]) s++;
		System.out.print(s);
	}
}