ORDER BY items must appear in the select list if SELECT DISTINCT is specified

I have SQL Server 2008R2 SSRS reports.

I'm using custom time dimension called Month_Year.
Reason for using Month_Year is that I would like to display month in format of 1/2013 to save space in parameter lenght.
Remember please that I'm using Interactive Sorting also. I have created special ORDER statement so that I get sorting correct.

I need help with following statement:

SELECT DISTINCT Month_Year
         .ProductName
         ,SUM(SalesAmont)
FROM Sales
GROUP BY Month_Year,ProductName
ORDER BY RIGHT(Month_Year,4) + '' + LEFT(Month_Year,1) DESC

I get following error:
ORDER BY items must appear in the select list if SELECT DISTINCT is specified.

THIS IS SELECT Month_Year FROM Sales ORDER BY Month_Year:
Month_Year
 6/2013
 6/2013
 6/2013
 6/2013
 6/2013
 6/2012
 6/2012
 6/2012
 5/2013 (this should be located after 6/2012 so I created new ORDER statement )
 5/2012
 4/2013
 4/2012
 4/2012

CREATE TABLE [dbo].[Calender](
 [Date] [datetime] NOT NULL,
 [MonthNumber] [numeric](2, 0) NOT NULL,
 [Month_Year] [varchar](7) NULL,
)

CREATE TABLE [dbo].[Sales](
 [Month_Year] [varchar](7) NULL,
 [ProductName] [varchar](7) NULL,
 [SalesAmount] [float] NOT NULL,

Kenny_I