Sum a Total with ReportItems in SSRS

My report is pulling daily info. doing some math with the textbox name fields.
That part works.

Now I need to do a Total at the bottom of the page for AVG and SUM.  I get this error.
when I try this expression.

=Sum(REPORTITEMS!B.Value+ReportItems!C.value)

the error is:
[rsAggregateReportItemInBody] The Value expression for the textbox 'textbox18' uses an aggregate function on a report item.  Aggregate functions can be used only on report items contained in page headers and footers.
[rsReportItemReference] The Value expression for the textbox textbox18 refers to the report item B.  Report item expressions can only refer to other report items within the same grouping scope or a containing grouping scope.
[rsReportItemReference] The Value expression for the textbox textbox18 refers to the report item C.  Report item expressions can only refer to other report items within the same grouping scope or a containing grouping scope.
Build complete -- 3 errors, 0 warnings

Please look over the screen shots and let know what I am doing wrong.
Sam
ReportItems.JPG
screenshot1.JPG
screenshot2.jpg