Leetcode Easy 2 Roman to Integer

class Solution:
    def romanToInt(self, s):
        # if s == '':
        #     return 0
        def compare(a, b):
            if dic[a] >= dic[b]:
                return dic[a]
            else:
                return -dic[a]
        dic = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000}
        sum = 0
        for i in range(len(s)-1):
            sum += compare(s[i], s[i+1])
        sum += compare(s[len(s)-1], 'I')
        return sum