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This function will return undefined (NULL in PHP) if any of the values aren't numbers.

Which I assume is None in Python.

I agree with this issue - getting the same error when attempt my solution.

The issue for me seems to be that I'm writing in python. And the tests will generate something like:
-79--73
in the list.

Which python will interpret as
-79 minus -73

So the list won't include "-79--73", but instead will have -6 in its place.