How Do I Put Records Number In Union

Hi i have sql statement like this :

SELECT row_number() over (ORDER by a.empid) as rec_num, empname
FROM employee_a
UNION
SELECT row_number() over (ORDER by a.empid) as rec_num, empname
FROM employee_b

the problem is the rec_num repeat for each statement like this :




rec_num empname

1 john
2 maggy
1 lee
2 mary
3 louis

How do i make the rec_num continue for the next statement after union.