Aggregate of an expression containing an aggregate

I'm converting a Crystal Report to an SSRS 2005 report. I have data similar to this:

ID    Amount    ChildID     Name    Discount
1     200       1           Billy   $10
1     200       2           Bobby   $20
2     100       1           Kenny   $10

I need to subtract the sum of the discount from the amount at the group row level. Then at the end I need to summarize those amounts. So it will look like this:

ID: 1 Amount: $170
    Billy      $10
    Bobby      $20
ID: 2, Amount:$90
    Kenny      $10

Total:        $260

I have the the grouping on ID in place and am calculating the amount with the following expression:

=FormatCurrency(First(Fields!Amount.Value)-Sum(Fields!Discount.Value))

How do I 1) get a total sum of these values to display in the footer and 2) use that sum in another calculation in the report footer? In the Crystal Report I solved this by using a global variable that kept a running total of the calculated Amount.