I need a hint to solve problem F (password remembering) at ICPC DHAKA 2009.

I have tried to solve this problem for several days unsuccessfully...

At this link you can get a description and some comments about the questions:

http://www.northsouth.edu/acm/dhaka2009.pdf

The link of the problem in Live Archive is:

https://icpcarchive.ecs.baylor.edu/index.php?option=com_onlinejudge&Itemid=8&category=346&page=show_problem&problem=2499

I would appreciate any help. Thanks in advance!