This is a repost of the blog post from before Black Day. I'm lazy, though, so it's just printscreened from Google Cache :D. It got less sharp, but no less readable. Now in an improved version! (still a picture :D)

I really liked this problem (394E - Lightbulb for Minister), because it involves taking derivatives. You don't see this much in programming contests :D. Therefore, I'd like to post my solution in detail. This algorithm works clearly in O(N + M). My AC code: 5931254