# Learn How to Work with Numbers and Strings by Implementing the Luhn Algorithm - Step 17

### Tell us what’s happening:

what is the exact index that I have to write on the index operator from “odd_digits”.

I also dont know what the instructions refere with “that contains every other digit of the card_number_reversed strin”

``````
# User Editable Region

def verify_card_number(card_number):
odd_digits = card_number_reversed[]
sum_of_odd_digits = 0
card_number_reversed = card_number[::-1]

# User Editable Region

def main():
card_number = '4111-1111-4555-1142'
card_translation = str.maketrans({'-': '', ' ': ''})
translated_card_number = card_number.translate(card_translation)

verify_card_number(translated_card_number)

main()
``````

User Agent is: `Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Safari/537.36`

### Challenge Information:

Learn How to Work with Numbers and Strings by Implementing the Luhn Algorithm - Step 17

Have you tried submitting an answer?
What error message do you get if you submit incorrect code?

Here’s a useful article which explains how slice string works in Python.

the message error says: Your code raised an error before any tests could run. Please fix it and try again.

[quote=“romo.emi19, post:1, topic:682900”]
odd_digits = card_number_reversed

in this line of code is the error

Ah, yes, that would raise an error, because you have empty brackets.
I’d recommend reading up on slice strings, as it’s an important concept to get your head around.

HINT: If, you’re still stuck, try removing the empty brackets and see what error message you get next time.