How to reply to this negative question?

In actual real life conversation, you're more likely to be understood if you leave of the yes/no part and simply make the statement. "I'm interested".

If you are asking from a precise logical standpoint, and question is "not interested" and if you are interested then the answer is no. But that's the sort of trick question that causes arguments with parents and girlfriends.

Just say "I am interested." Why do you need another word?

As you point, out, replying with either a simple "yes" or a "no" can be misinterpreted. So, if you want to be perfectly clear, reply with a sentence. Example: Question - Are you interested in coming with us?" Answer - Yes, I am interested (or "No, I am not interested.")

In theory you would say "No, I am not 'Not interested'" thus implying that you are interested. However in common parlance the expected answer would be "yes" if you were and "mno." if you weren't.

Maths time :)

So we all know to negatives equal a positive so to counter that not which is negative you need to say no, I'm not interested.

Although I often say yes out of habit