import java.util.*;

class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		
		int n = sc.nextInt();
		int[] ar = new int[n];
		for (int i=0; i<n; i++) ar[i] = sc.nextInt();
		int ans = 0;
		for (int i=0; i<n-2; i++) {
			if (ar[i]!=ar[i+1] && ar[i+1]!=ar[i+2] && ar[i]!=ar[i+2]) {
				if ((ar[i]<ar[i+1]&&ar[i+1]>ar[i+2]) || (ar[i]>ar[i+1]&&ar[i+1]<ar[i+2])) {
					ans++;
				}
			}
		}
		System.out.println(ans);
		
	}
}