Sequence Number For Records

Hi...

I have Sql statement more like this

SELECT row_number() over (ORDER by a.employeeID) as rec_num, a.* FROM EmployeeA a
UNION
SELECT row_number() over (ORDER by a.employeeID) as rec_num, a.* FROM EmployeeB a


rec_num employeeID employeeName employeeDepartment


1 777 Mike HR
2 888 Susy HR
1 111 Smith TECH
2 222 John TECH
3 333 Lenny TECH



How do i get sequence number for all of this records. The rec_num reset for every statement. I want the records numbering for second statement continue from first statement so that it can be like this :

rec_num employeeID employeeName employeeDepartment


1 777 Mike HR
2 888 Susy HR
3 111 Smith TECH
4 222 John TECH
5 333 Lenny TECH