Hi,
I have a formula here which uses only one text field for the first and last name. If you are using a separate field, kindly replace the formula for the 2nd concat value (last name) to the proper one.
concat(substring(field1, 1, 1), substring(field1, string-length(field1) - 3, 4))
where substring(field1, string-length(field1) - 3, 4)) is intended for the last name.
Please let me know if this helps.