Roman to Integer

Roman to Integer.py

class Solution:
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        sum  = 0
        last = None
        numbers = {"M":1000, "D":500, "C":100, "L":50, "X":10, "V":5, "I":1}
        
        for st in s:
            if last and  numbers[st] >last:
                sum  -= 2 * last
            sum += numbers[st]
            last = numbers[st]
        
        return sum