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);
	}
}



/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz