Oh, unfortunately I didn't know correct formula There was no such clarification on "Clarification board" or maybe I searched in a wrong place. But looking IRR in wikipedia could be a great idea.
Thank you.
I spent half of contest time on solving it but have no ideas how to deal with precision problems.
It's clear that answer can't be "No", but how to distinguish case with two close roots from case with exactly one root?
Thanks in advance.