Sequence number

I want to add a field in Ultimate member where is filled in automatically by the system with sequential code, for example: 0001, 0002, 0003, etc.
or at the prefix filled in automatically takes another field value. example:
there is a field “sex” with a value “Male” and “Female.”
Then the contents in the field are:
M-0001, M-0002, etc.
F-0001, F-0002, etc.

How can I achieved it?