Help with cell formula - Demand planning

Hi all
I have the attached excel sheet, and Im trying to work out the sum for cell F11
Cells D2 to D7 contain the number of each device I have in stock, in cells F2 to F7 I have the demand requirement for each of the line items
In cells D11 to D16 I have the number of widgets that need to go into each item
In cell E11 I have a manually enter number that I have in stock
In Cell F11 I want it to show a number that I would need to produce

In the attached example the calculation I have is
=SUM(D2-F2)+(D3-F3)+(D4-F4)+(D5-F5)+(D6-F6)+(D7-F7)
Giving an answer of 150

But I need to have an answer of 0 as I already have enough devices in stock and I also have the parts in stock to make the demand

This has been frustrating me for a few days so any help you can give would be greatly appreciated

Cheers

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