I think, it is one of most complicated problems in the archive. Several different ideas make search fast enough, but I never might have guessed it without tests below. So, thank you, DWED!

My program ran fast, even with little optimization..
But wrong answer on 7.. up till now, i haven't found any bugs from it..
Could you leave some hints for me?:( thanx.

See e-mail in my profile. It is very tiresome to hold such conversation in English.