Which of these is the proper form?

Hi,

in Poland at the end of highschool we have finals, which are the same for the whole country and are decisive when it comes to college/univerisity applications. This year's advanced english was rather easy, I encountered only one troublesome question.

Should it be:

... but unless you didn't know, you would never guess.

or

... but if you didn't know, you would never guess.

?

Thanks in advance,

Michael