how to add a special subtotal in the pivot table?

[MiaYang]

my excel look like this:

part qty part2
-----------------------------------------
p1 10 fg1
p1 10 fg2
p2 5 fg3
p2 5 fg4
p2 5 fg5
-----------------------------------------

the pivot table looks like this:

part qty part2
---------------
p1 10 f1
f2
p2 5 f3
f4
f5
---------------------------------------------

if i sum the instock_qty, it will look like this:

partnumber instock_qty usedpartnumber total
--------------------------------------------------
p1 10 f1 10
f2 10
-------------------------------------------------
p1 total 20
-------------------------------------------------
p2 5 f3 5
f4 5
f5 5
------------------------------------------------
p2 total 15
------------------------------------------------

But what i want is:

partnumber instock_qty usedpartnumber
---------------------------------------------
p1 10 f1
f2
---------------------------------------------
p1 total 10
---------------------------------------------
p2 5 f3
f4
f5
---------------------------------------------
p2 total 5
---------------------------------------------

how can i do that? really appreciate your help!