Page break in .rdlc subreport with a row number in Visual Studio 2010

Hi,

I have a problem with page break in rdlc subreport and I need help. I have a rdlc report include a main report and a subreport. I want to put a page break each 15 rows per page in subreport. I tried creating a group and setting the group expression by formula: System.Math.Ceiling((RowNumber(Nothing)-1)/15)+1; however, the result was not correct. I'm using Visual Studio 2010. Please help me the solutions to solve that problem. I need the solution urgently now. I hope receiving the answers as soon as possible.

Thanks very much.