import java.util.*;
public class kadomatsu{
	public static void main(String... args){
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int[] k = new int[n];
		for(int i = 0; i < n; i++){
			k[i] = scan.nextInt();
		}
		System.out.println(search(n,k));
	}
	public static int search(int n,int[] k){
		int count=0;
		for(int i = 0; i < n-2; i++){
			int low = k[i];
			int middle = k[i+1];
			int high = k[i+2];
			if(low!=high){
				if(middle > low && middle > high){
					count++;
				}else if(low > middle && middle < high){
					count++;
				}
			}
		}
		return count;
	}
}