Subtract Negative Numbers Within Pivot Table Data Field

Ive attached a spreadsheet showing what im trying to achieve. basically i have a scheduling objective, in which only a few slots (AA, BB, CC) are available. so the starting availability is fixed. for this example assume the starting values are 6,7, and 8 respectively. as 'bookings' are entered, im using the value of -1 against A, B, or C respectively, to indicate that AA, BB, or CC should be decreased by 1 unit of availability.

im trying to incorporate the subtraction within a pivot table. as you can see i am way off. the hypothetical formula logic that im trying to implement within the PivotTable would look something like this:

for A,B or C, sum all (-1) values for each instance of Date and Stage... this will give total bookings

add this total value to the starting values of AA, BB, CC respectively to get the new availability