I would like to know if one can describe the following as a formula;

(It's a converter from years to cat years)

First year is equal to 15 cat years, the second years is equal to 10 cat years and then every following year is equal to 4 cat years.
Bear in mind I don't want to describe it with like n > 2 and then the formula, I would like to now if there is a way to describe without having to limit it to only include for instance when "n" is bigger then 2.

Help appreciated!

(Sorry for possible misplacement, I wasn't really 100% sure where to post it, but if you can come up with a better place to post this I can delete this post and repost it at the correct place)