import java.util.Arrays;
import java.util.Scanner;
public class Main{
	public static void main(String args[])throws Exception{
		Scanner sc=new Scanner(System.in);
		int n=Integer.parseInt(sc.nextLine());
		int are[]=new int[n];
		for(int i=0;i<n;i++)
			are[i]=sc.nextInt();
		sc.close();
		int counter=0;
		for(int i=0;i<n-2;i++){
			if(JD(are[i],are[i+1],are[i+2]) && JC(are[i],are[i+1],are[i+2])){
				counter++;
			}
		}
		System.out.println(counter);
	}
	private static boolean JC(int a,int b,int c){
		int are[]={a,b,c};
		Arrays.sort(are);
		if(are[1] == a || are[1] == c)return true;
		else return false;
	}
	private static boolean JD(int a,int b,int c){
		if(a==b || a==c || b==c)
			return false;
		else return true;
	}
}