Enumerate rows in a SQL QUERY with Counter

Hello 
I have a query with UNION 

Select ‘input’ as Type, Name, City, date form Table1
union 
Select ‘Output’ as Type ,Name, City, date  from Table2
ORDER by date

As result, I would like to have a rownumber as part of Query 

1, Input, Peter, New York, Dec-01
2, OutPut, Joseph, California, Dec-03
3, Input, Mary, Atlanta, Dec-09

How can I add a ROW_NUMBER to the query?

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Carlos Pelaez