Cost calculation in Purchase Order

Hi,

I have a scenario. where i want the cost of an item in purchase order to be calculated as sum of cost of other items in purchase order.

for example- in a Purchase order

Line 1 - item1 = unit cost = 5

Line 2- item2 = unit cost = 10

Line 3- item 3 = Unit cost = 8

All of these cost of item1, item2, item3 are coming from F4105

But i want that cost of item3 on line 3 = cost of item1 + item 2 = 15 instead of 8

how i can achieve this without updating it manually.

Regards,

Nik