What's wrong with this code?

[kumaaaa]

This is a code i made that checks whether an isbn with 13 digits is valid, but there is something wrong but I don't know what it is.
The principle is like this
for example, 978-0-306-40615-? is calculated as follows:

sum = 9×1 + 7×3 + 8×1 + 0×3 + 3×1 + 0×3 + 6×1 + 4×3 + 0×1 + 6×3 + 1×1 + 5×3
= 9 + 21 + 8 + 0 + 3 + 0 + 6 + 12 + 0 + 18 + 1 + 15
= 93
93 / 10 = 9 remainder 3
10 – 3 = 7
Thus, the check digit is 7, and the complete sequence is ISBN 978-0-306-40615-7.
if the sum is divisble by 10, the remainder will be zero and 10-0=10,
however, the check digit is still 0.

However in the code, when I check 9781491939369, which is a valid isbn, it outputs invalid.

isbn=input('ISBN: ')
sum=(1*int(isbn[0])+3*int(isbn[1])+1*int(isbn[2])+3*int(isbn[3])+1*int(isbn[4])+3*int(isbn[5])+1*int(isbn[6])+3*int(isbn[7])+1*int(isbn[8])+3*int(isbn[9])+1*int(isbn[10])+3*int(isbn[11]))
if sum%10==0:
    if isbn[12]==0:
        print('Valid')
    else:
        print('Invalid')
else:
    if isbn[12]==10-(sum%10):
        print('Valid')
    else:
        print('Invalid')

Thanks a lot.