ldap - C# DirectorySearcher filter -

i trying fetch records ldap using directorysearcher. following filter using:

(&(objectcategory=person)(objectclass=user)(employeetype=a))

i need modify filter fetch records employeetype either or b.

please help.

in expression want replace employeetype=a employeetype=a or employeetype=b

(&(objectcategory=person)(objectclass=user)(employeetype=a)) 

so replace (employeetype=a) (|(employeetype=a)(employeetype=b)) , get

(&(objectcategory=person)(objectclass=user)(|(employeetype=a)(employeetype=b)))