As a non-rated contestant, this is my first unrated round.

Great. You won't be waiting till the end of a contest to practice virtually :)

I have used 1e13 lol, just break the loop as soon as sum becomes GTOE health. I mean if you're not breaking the loop, it will cause overflow .

Consider 2e5 values equal 2e5 in array a, array c : 2e5 values equal 1 .. so, when multiplying (mid / c[i] * a[i]) 1e12 * 2e5 * 2e5 gives 1e22 which doesn't fit into 64bit :)

Same wondering about a = 4 & b = 5, till I returned back to the problem and read it again :)

What the he*ll bores?

it has been postponed for 10 mins