Help create a dield with code

Hi
I would like some help to make some fields in Crystal Reports and print them in my form.

I have a table with the above fields:

Type
Charge
CustomName


So for example for one customer i have
Type Charge CustomName
1) 900 61,50 Maria
2) 900 61,50 Maria
3) 901 23,00 Maria
4) 900 61,50 Maria

Now i want to make first a field:
(Pay) that will make the sum if type is 900 or remove if 901 and first will be 0
1) 61,50
2)123
3)100
4)161,50
How i can do that with code??? so print my field in my report