Try the last problem from here: https://cemc.math.uwaterloo.ca/contests/computing/2009/stage1/seniorEn.pdf.

Thank you so much zxqfl, I didn't know that problem, and it's very interesting :). Anyway, I've been trying to solve it for a few hours, and always receive TLE. I've made the calculus, and should pass (tight, but still pass). Surely my approach isn't the correct one, and I'm thinking something wrongly. Here's my code. Basically what I did is update the first element a circle touches in each row, and the next element of the last one that a circle touch (range update, like in Fenwick Tree), then traverse all the grid updating each cell and saving the maximums. It should be O(n*m*3), around 9x10⁷ operations... I don't know, could you give me some clue about the solution? haha And thanks in advance :)