class Calc0116
  def initialize(args)
    args = args.map { |l| l.chomp.split(/\s+/) }
    @n = args.shift.first.to_i
    @as = args.shift.map(&:to_i)
  end

  def run
    (1..(@n - 2)).count { |i|
      as = @as[(i - 1)..(i + 1)]
      as.uniq.size == as.size &&
      [0, 2].include?(as.index(as.sort[1]))
    }
  end
end

puts Calc0116.new(STDIN.readlines).run if __FILE__ == $0