getting error in function sql server 2008

ALTER FUNCTION [dbo].[FNGetDataRowCount](@DatasetID AS INT)  
RETURNS BIGINT  
AS  
  BEGIN
  	

  DECLARE @retval AS BIGINT
  DECLARE @temp TABLE (
 
                table_name sysname ,  
                row_count INT ,
                reserved_size VARCHAR(50),  
                data_size VARCHAR(50),  
                index_size VARCHAR(50),  
                unused_size VARCHAR(50)
                )  
                  
                DECLARE @datasetName varchar(100)  
                SET @datasetName = '['+Cast(@DatasetID AS varchar)+']'  
                  

INSERT @temp  
    EXEC sp_spaceused @datasetName  

 SELECT @retval=row_count from @temp  WHERE table_name=@DatasetID
 
 
 RETURN @retval
 
  END

Msg 443, Level 16, State 14, Procedure FNGetDataRowCount, Line 22
Invalid use of a side-effecting operator 'INSERT EXEC' within a function.