(Invoice) Number and Letters in progressive billi

-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.4.3

Good morning, my version is 2.5.0 hikashop business with Joomla 3.4.3
The question is this: how to enter the number of billing sequential numbers and letters? For every order I wish numbering in billing was this: A1, A2, A3, A4, A5 in progression up to year-end, then starts over again with A1, A2, A3, A4, ECC. So instead of {automatic_code} code that I have to put it to work? I hope I explained, thanks in advance. I hope you can answer me as soon as possible, thanks

<edit>
Aaah, Nicolas' post here below says it so much better... and correctly, of course!

Hello thanks for the quick response so nice!
I currently use this:
Formato Numero Ordine: {id size="5"}/{date format="m/Y"}
Formato numero fattura: {id size="5"}/{date format="m/Y"}

would be enough to make both orders and invoice this:
Formato Numero Ordine: A{id}/{date format="m/Y"}
Formato numero fattura: A{id}/{date format="m/Y"}

rightly so ?

you can do this?

Formato Numero Ordine: A{id size="5"}/{date format="m/Y"}
Formato numero fattura: A{id size="5"}/{date format="m/Y"}

Thank you in advance

you can do this?

Formato Numero Ordine: A{id size="5"}/{date format="m/Y"}
Formato numero fattura: A{id size="5"}/{date format="m/Y"}

Thank you in advance

Then I would say that it is perfect! Thank you so much for availability and fast response.
Sorry for the double post

Thank you very much