class Octal
  attr_accessor :octnum

  def initialize(num)
    @octnum = num
  end

  def not_valid?
     octnum == '0' || octnum =~ /[A-Fa-f8-9]/
  end

  def to_decimal
    sum = 0
    return sum if not_valid?
    oct_reverse = octnum.reverse
    for i in 0...oct_reverse.size
      sum += oct_reverse[i].to_i* 8**i
    end
    sum
  end
end