I've been grinding 1900-rated problems lately and got completely walled by 1909I.

https://mirror.codeforces.com/problemset/problem/1909/I

I spent way too much time on it, finally checked the editorial and the solution involves advanced polynomial tech like NTT. I checked clist and it's rated around 4000.

Is the rating just completely bugged? If so, admins please fix it. But if the 1900 rating is actually correct, could some high-rated coders explain the simpler way to think about this?

Thanks!