i am using crystal report 10 and sql server 2000
there is a field name sale in database now
i just want to add two field in crystal report
one is sale which is stored in databse
second balace which shows the balance by adding
all the previos sale

like

sale========Balance
100========100
200========300
50=========350
10=========360
100========460
and so on

i know how to dislplay the filed even using filter
data but how to calculate balance at run time
is making problem for me
can any body help me is this reg