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