Sum Function in Expression with two Grouping fields in SSRS 2008

I have data values like below and I am going to find the sum between them based on two grouping.

Date------------City -------- Amount

1/7/2012          Omaha        4000

1/7/2012        Denver           5000

2/7/2012        Denver           1000

I need to have Sum(Amount) with two groups on Date as parent and City as child. it seems that Sum function handle just one group like below.

Sum(Amount,"Date").

How can I add "City" to the sum as well?

Thanks