Using ROW_COUNT

Hi Below is the code for my stored procedure


Alter proc [dbo].[InvoiceDisplayTest] (@InvoiceNumber nvarchar(50),@InvoiceDate datetime ,@InvoiceDate1 datetime, @AccountNumber nvarchar(50),@TradingPartnerID nvarchar(50),@Page nvarchar(50),@rownum int OUTPUT)
AS
BEGIN
SELECT * FROM (SELECT InvoiceID,DocumentNumber, convert(char(12),DocumentDate,112) AS DocumentDate,AccountNumber,Supplier,TradingPartnerID,


ROW_NUMBER()OVER(ORDER BY DocumentNumber ASC)AS rowno FROM InvoiceHeader

WHERE (@InvoiceNumber = '' OR DocumentNumber = @InvoiceNumber)
and
(@AccountNumber = '' or AccountNumber = @AccountNumber)
and
(@TradingPartnerID = '' or TradingPartnerID = @TradingPartnerID)
and
(@InvoiceDate = '' or DocumentDate >= @InvoiceDate
and
DocumentDate <=@InvoiceDate1 or @InvoiceDate1 = '')
)As qr
WHERE rowno BETWEEN ((@Page - 1) * 20 + 1)
AND (@Page * 20)

END


My Question is how do I SET my @rownum OUTPUT parameter equal to the total rows for the inner select statement. As the rows for the whole query on counts the ones being displayed. Cant i just use the alias of my ROW_NUMBER() to get the total rows. Or another way.
Looking foward to hearing from you

Mark