Invoice Legal Number invalid in PERU

HI all,

We have a problem when we try to add an invoice. In this case, our invoice come from electronics invoices, so we need to start the invoice number with "F", because goverment says the following:

"The serial is four-digit alphanumeric begining with the letter "F""

And when we try to add in JDE, it shows the following error:

A form level error has occurred.

CAUSE

Invoice Legal Number has special characters or invalid format.

RESOLUTION

Change this field and enter a valid data.

Is there any way to start invoice numbers in Peru with "F"?

Thanks in advance.