Select every Nth element or N element input

There is list of things, for example books. User selects which books should be highlighted. We do not know how many books there are, so user uses index, for example:

• The second book should be highlighted. (#2)
• The 10th book should highlighted. (#10)
• Every sixth book should be highlighted (#6, #12, #18 ...)
• Every 20th book should be highlighted (#20, #40, #60, #80 ...)

I'm trying to design an input that should define this index, so it's clear to user what it does.

The best idea that I've come up so far is to combine dropdown and simple number input (so dropdown chooses if it is "second" or "every second"):

But it still feels wrong. Any tips on how to make it better?

Thanks!