How to Get the Maximum of Headers Summary in Crystal Report

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VB

CrystalReports

Hi! I have a problem in Crystal Reports on VB.NET 2008. I have two GroupHeaderSection, one is grouping per Department and the other is per Section. Under the Section grouping, I have series of Summaries, i.e. EmpA, EmpB, EmpC. These summaries are the Sum of all the workOutputHrs of employees per section (workOutputHrs is from an SQL table, saved per date). The Department & Section are from TableA while the workOutputHr is from TableB. These two tables are linked by the EmployeeID (auto). What I want to do is to get the Maximum values of these summaries and put it in the GroupFooter. I tried this code but I get "This field cannot be summarized":

Maximum (Sum ({TableB.EmpA}, {TableA.Section}))

I hope someone can help. Thanks!