conditional access to "final exam"

Hi,

I am sure this has been asked before, but I can't find that piece of knowledge I am missing.

I need to restrict access to the "final exam" in my course.

The learner should be able to access the "final exam" slides only if he/she has visited all the previous "lesson" slides.

Because my course is long, learners should be able to view the "lesson" slides over multiple sessions.

I think I understand how Storyline variables work, but I need a global variable that is persistent ( a cookie?). That variable would read "true" is the learner has seen all the "lesson" slides (even over multiple sessions) or "false" is at least one "lesson" slide has been missed.

Any idea how to do that?

Thank you in advance,

Vanya