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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.
It's been six months since the question was asked, but in case someone else runs into this problem, here's what's going on. The tests check for negative numbers, and you're likely treating '-' as a digit of the number somewhere. Since '-' isn't a number/int, it gives this error. The instructions should point out negative numbers must be accounted; though really it doesn't make sense to include them given that the output can't be negative.