import java.io.*;
import java.util.*;
import java.math.*;

class Main {
	
	public static void out (Object o) {
		System.out.println(o);
	}
	
	public static boolean isPd (int a , int b , int c) {
		if (a == b || a == c || b == c) return false;
		return (c < a && a < b) || (b < a && a < c) || (a < c && c < b) || (b < c && c < a);
	}
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(br.readLine());
		int[] a = new int[n];
		String[] line = br.readLine().split(" ");
		
		for (int i = 0; i < n; i++) {
			a[i] = Integer.parseInt(line[i]);
		}
		int ans = 0;
		for (int i = 2; i < n; i++) {
			ans = isPd(a[i - 2] , a[i - 1] , a[i]) ? ans + 1 : ans;
		}
		
		out(ans);
		
	}
}