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

public class Main {

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