hi there!, hope somebody can help me, i want to get the running balance of my yearly budget like this

Budget Full Year: 2,284,500.00
total expenses on january: 160,000.00
BUDGET BALANCE: 2,124,500.00

total expenses on february 100,000.00
BUDGET BALANCE: 2,024,500.00

i was trying a formula code like this:

Code:
numbervar budget := budget - totalexpenses;
i placed it in the group section,

but it wont work, anybody there who knows this? thank you.