[Urgent]Calculating percentage in pivot table

Hi all,

I am new to excel pivot table and I would like to ask one question about pivot table:

I have maded a pivot table which the rows show month (e.g. 1,2... 12), the columns show name of staff( e.g. Peter, Marco etc), a logic indicator (either 1 or -1), the filter is year (different years like 2012,2013,2014), the values is using count numbers where it counts the number of data that belongs to 1 and -1 respectively.

Now I want to create a column in pivot table which calculates the following:

(Total number of data that belongs to 1)/( (Total number of data that belongs to -1) + (Total number of data that belongs to 1))*100

How I can deal with it?

Thank you so much!