leetcode 13 -> Roman to Integer

class Solution(object):
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        lista={'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}
        Value_sum=0
        last_Value=0
        for i in range(len(s)):
            if (lista[s[i]] in [1,10,100]) and ( last_Value not in [1,10,100]):
                Value_sum=Value_sum+lista[s[i]]
            elif (last_Value in [1,10,100]) and (last_Value<lista[s[i]]):
                Value_sum=Value_sum+lista[s[i]]-2*last_Value
            else:
                Value_sum=Value_sum+lista[s[i]]
            last_Value=lista[s[i]]
        return Value_sum