Language / Rozumiesz - I have a question on Polish language. [11]

Dzień dobry!
I'm new on here. I'm learning Polish and I'd like to ask a question.
I am not sure how to answer the following question. Am I correct?

***
Don't you understand? - Yes, I understand.
Don't you understand? - No, I don't understand.

And this is my choice.
Nie rozumiesz? - Nie, ja rozumiem. or Tak, ja rozumiem.
Nie rozumiesz? - Nie, nie rozumiem.
***

Could someone please explain to me?
Thank you in advance for the answer :)