I have done it!
The Frank's Mystery was solved :)
I used integrals, complex variables, analytic geometry and advanced mathematics... it's joke :)
I really solved this puzzle. And I'll give a hint at the end of this commentary.
= My review
This is not a monotonous game. Shuinvy came up with various quests.
And I was pleased with the right attitude to children at the game end :)
Thank to Shuinvy and to Davis Meleod (who help translate the game to the English, his name is written on the end-titres)
= The hint of the The Frank's Mystery
...look only in the FRANK- book, only at numeral.
And mentally do something with it.
(there are all the necessary information for four-digit password)