creating a summary of the summary is not possible using the built in function. However you can create  a formula which can perform the sum.

Unfortunately, you can't quite do it that way.

Instead, add a formula to each group footer, that simply returns the SUM for that group.  Then, take the Maximum of that formula in your report footer.

As a note, this doesn't always work.  This is because of the order in which Crystal performs various operations.  One way to help Crystal perform everything in the correct order is to put all your summary calculations in the footers, instead of the headers.