import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		final int N = sc.nextInt();
		
		int[] arr = new int[N];
		for(int i = 0; i < N; i++){
			arr[i] = sc.nextInt();
		}
		
		int count = 0;
		for(int i = 1; i < N - 1; i++){
			if(arr[i - 1] < arr[i] && arr[i] > arr[i + 1] && arr[i - 1] != arr[i + 1]){
				count++;
			}else if(arr[i - 1] > arr[i] && arr[i] < arr[i + 1] && arr[i - 1] != arr[i + 1]){
				count++;
			}
		}
		
		System.out.println(count);
	}
	
}