Ok, I've already searched this site. There are questions about this. I've read them and read the answers. But they don't really answer what I wonder. I'm asking about grammar.
Let's say the cashier at the counter says "How are you doing?". If I answer with only "Good.", I feel like it's so abrupt and rude. If I answer it with "I'm good, thank you.", I feel like it's not right, grammatically (I don't want the other to judge me as I'm a freak).
Because according to grammar, that question needs to be answered with an adverb; and "I'm good" is totally nothing relates to that. What is the general, proper, short reply in this situation, and can "good" really be used as an adverb instead of "well" in everyday informal speech?