Better user experience when asked to input frequency of periodic event

So I am working on a form and one of the inputs have proved in user-testing to be somewhat problematic:

We want to save the frequency of a periodic event, the event is how often a report is to be submitted, in our database we will save a numeric value that is describing times-per-year.

It can be both more and less than one time per year, so in the event of every fourth year, the value we want to save is "0,25" and for four times per year it is "4".

In a previous form we asked the user to calculate the number themselves, in use this has proven to be difficult for the users.

We have tested a solution with two input fields, one for the frequency and one for number of year: X times per Y years (where X and Y is the input fields.) In a user-test with 4 users, we had only one solve this relatively without issues and 2 did not solve it at all when asked to input "once every second year".

Any suggestions for other ways to have the user input the requested data?

Thanks, Roy