using System;
using System.Collections.Generic;
using System.Linq;

public class Yuki116
{
	public static void Main()
	{
		var n = int.Parse(Console.ReadLine());
		var a = Console.ReadLine()
				.Split(' ')
				.Select(x => int.Parse(x))
				.ToList();
				
		int ans = 0;
		for(int i = 0; i < n - 2; i++)
		{
			ans += Check(a[i], a[i + 1], a[i + 2]);
		}
		
		Console.WriteLine(ans);
	}
	
	public static int Check(int x, int y , int z)
	{
		if(x == y|| x == z || y == z) return 0;
		if(Math.Max(x , Math.Max(y , z)) == y || 
			Math.Min(x , Math.Min(y , z)) == y) return 1;
		return 0;
	}
}