How to limit to a response to be a multiplier?

Hello, I would like to set up a numerical free text question on "how many loyalty rewards point to redeem?". For which the answer must be a 500 multiplier e.g. 500, 1000, 1500 and so on (may be up to million). Is there any way I can limit the respondent to provide the answer to be a 500 multiplier on the free text answer field? I have searched all around but it seems nothing available. I have tried to validate the answer by limiting last three digits to be either "000" or "500" by match regex. But I am sorry I have zero knowledge on this field, so any helps on this will be very much appreciated. Thank you so much for all your attention to this question!!