Conversion from Roman numerals in Dive into Python seems to output an extra character

Question

I am learning Python 2.7 by Dive to Python. Here are the codes of "Converting between Roman Numerals and Arabic numerals":

romanNumeralMap = (('M',1000),
        ('CM',900),
        ('D',500),
        ('CD',400),
        ('C',100),
        ('XC',90),
        ('L',50),
        ('XL',40),
        ('X',10),
        ('IX',9),
        ('V',5),
        ('IV',4),
        ('I',1))
def toRoman(n):
    result = ""
    for numeral, integer in romanNumeralMap:
        while n >= integer:
            result += numeral
            n -= integer
    return result

def fromRoman(s):
    result = 0
    index = 0
    for numeral, integer in romanNumeralMap:
        while s[index:index+len(numeral)] == numeral:
            result += integer
            index += len(numeral)
    return result

print toRoman(1356)
print fromRoman('MCMLXXII')

But I am puzzled by the procedure of this part:

for numeral, integer in romanNumeralMap:
        while s[index:index+len(numeral)] == numeral:
            result += integer
            index += len(numeral)

When executed in the second time, the s[index:index+len(numeral)] returns CM, but I think it should return C, because the index = 1 after the first execution, then index+len(numeral) = 2, and s[1: 2] = 'C'. What's wrong with my understanding?

Answer 1

On the second iteration of the for loop, index is indeed 1
numeral is CM and integer is 900

len(numeral) == 2, and

s[1:1+2] == S[1:3] == 'CM'