Sorry for my monkey stupidity :D you're absolutly right, there is big O problem I wanted to say

Isn't that O(n)? Never heard of N(O).

I don't think this comment is spoiler coz there is the same hint in the link below kata's description. It directly points out to use this method in it's formula :)

And what if it's type become something more than a long?

Wow, Math always allows you to find the solution faster.