autonumber column exceptions based upon a checkbox
I have an auto number column that I use to assign an employee number to new employees. From time to time we re-hire a termed employee, in which case we do not need a new number - we would just use the old. I have included a checkbox if the employee is a rehire. My questions is - how can I have the auto-number column not assign a number to those employees? I would like to skip over them and continue numbering with the next new employee.