Getting Previous Values of Formula Fields in Crystal Report

Suppose if I want to add balance of a previous record to next like below ,

Amount	Paid	Balance
1000	500	500
1250	1000	750 (500+250)
500	300	950 (750+200)

>> First create a formula field, say: tempBalance


In that write:
shared numberVar x:=0;
x:={@balance} //{@balance} is formula filed which will calculate balance

In @balance write:
shared numberVar x;
{PaymentDetails.Total}-{PaymentDetails.Paid} + x

//In @balance the variable x will get the previous value from @tempBalance . Only we have to declare here as shared numberVar x;