Multiple arithmetic calculation in single tag

Hi,

I have a requirement where i need to subtract two values and multiply the result with another value. i.e.[(x-y)*z]

i am doing the calculation in below manner:

<?xdofx: (INITIAL_DURATION-INCREMENT)*invoice/DEVICES/SERVICES/AR_ITEMS/EVENTS/TOTAL/AMOUNT?>

Now as i have to use a for-each loop and the tag values in the loop will change so i used the below code:

<?xdofx: (INITIAL_DURATION-INCREMENT)*;./TOTAL/AMOUNT?>

Now, in the RTF if i remove the 1st code then the 2nd code is not working, i am attaching the RTF and a sample XML, can any one suggest a solution.