Employees at a logistics company are classified as either office-based staff (S) or field drivers (D).

Office-based staff (S) coordinate shipments inside the main depot.

Field drivers (D) deliver cargo across the country.

Employees must have exactly one of these roles; they cannot hold both roles simultaneously, but must hold one to be an active employee.

Construct a logic statement, using only the operators AND, OR, and NOT along with the letters S and D, to represent the conditions for being an active employee.