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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.
While dealing with negative values in this problem is indeed arguable, now appropriate note has been added to the description.
You always have to deal with wrong input values, even if they don't make sense. At my university quarter of all test cases are just handling bad inputs.