How to find sum

[RoniDxb]

AccountNo.....Date........Amount
1.............1/25/2008...500
1.............1/26/2008...500
1.............1/27/2008...500
1.............1/27/2008...500
1.............1/28/2008...500

I have a table named Table1 that has following structure.
I take three parameters from users as Starting Date , Ending Date and Account No
for example if user give the follwing parameters
Account No = 1
Starting Date = 1/27/2008
Ending Date = 1/28/2008

result of Report
AccountNo.....Date........Amount
1.............1/27/2008...500
1.............1/27/2008...500
1.............1/28/2008...500


Total Amount = 1500

Now the problem is i want to show the last previous Sum of this account too
for example in this case previous Balance will be

Previous Balance = 1000

plz guide me how can i take out prevous balance in Crystal Report