```
class Solution:
def romanToInt(self, s):
"""
:type s: str
:rtype: int
"""
i = len(s)-1
summation = 0
romanMap = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000,'IV':4,'IX':9,'XL':40,'XC':90,'CD':400,'CM':900}
while i >=0 :
currentDigit = s[i]
if (i == 0):
summation += romanMap[s[i]]
else:
beforeDigit = s[i-1]
double = beforeDigit + currentDigit
if double in romanMap:
summation += romanMap[double]
i -= 1
else:
summation += romanMap[s[i]]
i -= 1
return summation
```