I tried to solve it with a stupid exhaustive search, but for 99999999 it will be very much!!! Is there the formul with which I can solve this problem?

My code is very very faster than yours.But I don't know it's
problem.I think it is correct.
http://acm.timus.ru/forum/thread.aspx?space=1&num=1150&id=24037&upd=634082632672106250