roman numerals to dec

def roman_numerals_to_decimal(strParam):

  # code goes here
  roman_numerals_order = ['I', 'V', 'X', 'L', 'C', 'D', 'M']
  roman_numerals_mapping = {
    roman_numerals_order[0]: 1,
    roman_numerals_order[1]: 5,
    roman_numerals_order[2]: 10,
    roman_numerals_order[3]: 50,
    roman_numerals_order[4]: 100,
    roman_numerals_order[5]: 500,
    roman_numerals_order[6]: 1000,
  }

  def is_substractable(last, first):
    # roman_numerals = roman_numerals_mapping.keys()  # order not guaranteed
    diff = roman_numerals_order.index(last) - roman_numerals_order.index(first)
    if diff == 1 or diff == 2:
      return True

  if len(strParam) == 1:
    return roman_numerals_mapping[strParam]

  retval = 0
  chars = list(reversed(list(strParam)))
  skip_next = False
  for i, char in enumerate(chars):
    if skip_next:
      skip_next = False
      continue

    if i+1 < len(chars):  # still have one char left
      next_char = chars[i+1]
      if is_substractable(char, next_char):
        retval += roman_numerals_mapping[char] - roman_numerals_mapping[next_char]
        skip_next = True
      else:
        retval += roman_numerals_mapping[char]
    else:
        retval += roman_numerals_mapping[char]

  return retval