Sum textbox value in footer

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SSRS

SSRS-2008

SSRS-2005

Greeting
i have 3 textbox value in footer of my report
i want make total (sum) for tow of them
so when i try to make total with them give me this error

The Value expression for the textrun ‘.Paragraphs[0].TextRuns[0]’ refers to more than one report item. An expression in a page header or footer can refer to only one report item.

i used those code for sum but none of them working

=ReportItems!Textbox62.Value+ReportItems!Textbox61.Value

is there any idea fro solve this problem

What I have tried:

i also tried this but even give me error

=CDec(ReportItems!Textbox62.Value)+CDec(ReportItems!Textbox61.Value)