Doubt in exception aggregation

Hi Gurus.

I'm having a problem to create a specific report here. To simplify.

I have a .CSV file similar to:

0GL_ACCOUNT; 0FISCPER;ZBALANCE;

0011025002; 0032009; 10;

0011025002; 0032009; -10;

0011025002; 0032009; 10;

0011025002; 0042009; 10;

0011025002; 0042009; 10;

0011025002; 0042009; 10;

0011025002; 0052009; -30;

And the data above flows to an InfoCube.

I want to make a report with the balance for each fiscper. And the balance have to be cumulative. For example:

Mar/2009 Apr/2009 May/2009

Account 0011025002 10 40 10

The balance at the end of march is +10. Adding the balance of april, the total goes to 40, and adding balance of may, it decrease to +10 again...

I'm working with (in ZBALANCE key figure):

- Aggregation = SUM

- Exception Aggregation = Last Value

- Agg. Reference Char. = 0FISCPER

But it doesnt work. It dont acumulate the balance from last month with the result of the actual. The report show the balance of each month separately.

Does anyone know how can I make a report like the example above?

Regards,

Thiago