Loading collection data...
Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Paste your code here, mark your post as having spoiler content. I've solved it in Python and it worked fine for me (and for the other 6933 persons before).
Your code must be wrong, are you printing instead of returning the value?
None is the value your function returns and 13 is the expected value in that message.