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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.
With 0 having infinite factors, I am not sure what a correct output would be so I will change description so 0 should return -1 as well.
edit: test case added too