NUMBER_WORDS = {
      'zero' => 0, 'one' => 1, 'two' => 2, 'three' => 3, 'four' => 4, 'five' => 5,
      'six' => 6, 'seven' => 7, 'eight' => 8, 'nine' => 9, 'ten' => 10, 'eleven' => 11,
      'twelve' => 12, 'thirteen' => 13, 'fourteen' => 14, 'fifteen' => 15, 'sixteen' => 16,
      'seventeen' => 17, 'eighteen' => 18, 'nineteen' => 19, 'twenty' => 20, 'thirty' => 30,