input: Write a python function to convert Roman numerals to integers.
  assert convertRoman('IV') == 4
  assert convertRoman('IX') == 9
  assert convertRoman('MCMXCIV') == 1994
output: def convertRoman(s: str) -> int:
      roman_to_int = {
          'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000
      }
      total = 0
      prev_value = 0
      for char in reversed(s):
          value = roman_to_int[char]
          if value < prev_value:
              total -= value
          else:
              total += value
          prev_value = value
      return total