Display row count of dataset after grouping in report header?

My dataset has 500 rows, but if I group that data by field DeptId, there are only 20 rows.

I used something like the following, but it tells me that I can't use RowNumber in the page header:

=RowNumber(Fields!DeptId.Value, "DataSetTeachers")

I also tried something like CountRows("DataSetTeachers"), but that give me the total rows, which isn't what I want.

Thanks.