Very smart math logic!
Thank you for such a nice sample for my collection!

My opinion is:
That simple logic with a three numbers #include is too heavy an operation.
if-else would be much faster.

I beleave int m should be created out of (before) the for cycle.

This is a task that, like all tasks here, requires you to provide a valid solution, whatever it is. If an author doesn't want some library to be used, it's up to him to make it impossible.

That's IT !!!

I beleave This is not a task for using standard library from STL. I thnk they wanted us to create our own logic.