Trigger triggered from a foreign key

I have this trigger that works fine. The trigger prevents the deletion
of the last record.
But I want skip this trigger execution when the delete is done from a
external key.
How can I do this?

This is the fk
ALTER TABLE focgdepartment
ADD CONSTRAINT fk_focgdep_idfocg FOREIGN KEY (idfocg)
REFERENCES focg (id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE CASCADE;

This is the trigger
CREATE FUNCTION check_focgdepartment_delete_restricted() RETURNS trigger
AS $check_focgdepartment_delete_restricted$
BEGIN
IF ( (SELECT count(*) FROM focgdepartment WHERE idfocg =
OLD.idfocg)=1)
THEN RAISE EXCEPTION 'Last record can not be deleted';
END IF;
RETURN OLD;
END;
$check_focgdepartment_delete_restricted$
LANGUAGE plpgsql;
CREATE TRIGGER focgdepartment_delete_restricted BEFORE DELETE ON
focgdepartment FOR EACH ROW EXECUTE PROCEDURE
check_focgdepartment_delete_restricted();

Thank you