罗马数字转化问题 Python

输入的是3个I

为啥会出这种错误，

sum = 0
dic ={'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}
list_s = list(s)
i = 0
while i<=len(list_s)-1 :
if dic.get(list_s[i])>=dic.get(list_s[i+1]):
sum = sum + dic.get(list_s[i])
i=i+1
else:
sum = sum +dic.get(list_s[i+1])- dic.get(list_s[i])
i=i+2
if dic.get(list_s[-2])>=dic.get(list_s[-1]):
sum = sum +dic.get(list_s[-1])
print(sum)