You could say the same about any language

Coding in C# is like coding a suit, everything looks neat and tidy if you follow the conventions and standards. Otherwise your code will look like a fat cat.

A detailed comment isn't necessary, just some mild familiarity with Linq and the string constructor.

C# is not a good code golf language

Really clever, but a bit hard to read. A few line breaks and spaces and it would be better. For performance of course not the best solution but from effort.

I like how peoples solving problems in one line thinking its a good practice, this is a nightmare for maintenance if you asked me, and this line is totally NOT CLEAR, a very detailed comments is required to understand whats going on in there for someone who doesn't know the context .

The instructions state you won't get negative numbers on input, therefore IMO it's like a contract and you really don't need to count with negative numbers in this case. This is why the kata tests also pass because they won't send you any single negative number. Also check the similar answer from @Chrono79 below.

The exercise instructions clearly state this, which makes it a requirement.

This is correct since the acceptance criteria don't expect negative numbers as input parameter. We are not trying to do classic sad path validation.

Not exists validation for negative numbers