This solution is accepted: http://mirror.codeforces.com/contest/479/submission/8546349

However, if I change line 29 from

ps[j] = ps[j - 1] + dp[pidx][j];

to

ps[j] = (ps[j - 1] + dp[pidx][j]) % MOD;

The outputs of test 6 and many other test cases of my program are wrong. I have two questions:

1. I think that the two versions are mathematically equivelent, aren't they?
2. Why overflow error does not happen for the first version? Unsigned long long is really large enough?

Thanks for your help.

I don't think so. But why there is a β sign at the top right hand corner of the logo?

I received the email about the competition at April, 23, 2014 00:04 UTC+8 (April, 22, 2014 16:04 UTC), but the event started at April, 22, 2014 20:00 UTC+8 (April, 22, 2014 12:00 UTC). Why did I receive the email 4 hours and 4 mins AFTER the competition? Is it my own problem? (I use gmail)

"It will be rated events for all the participants. Do not miss the round!"

And surely I have missed the round :(

UPD: I see that some other users have the same problem: http://mirror.codeforces.com/blog/entry/11872 I am not alone :)