input: Write a function to convert a Roman numeral to an integer.
  assert roman_to_int('IX') == 9
  assert roman_to_int('III') == 3
  assert roman_to_int('MCMXCIV') == 1994
output: def roman_to_int(s: str) -> int:
      roman_values = {'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_values[char]
          if value < prev_value:
              total -= value
          else:
              total += value
          prev_value = value
      
      return total