Update inventory worksheet by adding/subtracting stock with the quantity.

ATTACHMENT has
inventory.sheet (inventory list)
incoming.sheet (add stock to inventory)
outgoing.sheet (subtract stock from inventory)


Can someone show me how to update 'inventory.sheet' sheet by adding stock in 'incoming.sheet' and subtracting stock in 'outgoing.sheet'

'incoming.sheet' and 'outgoing.sheet' consists of two columns. First column will have the ItemNum and the second column will have the Quantity. If the Quantity is empty beside a ItemNum, then assume it is "1"

'incoming.sheet' and 'outgoing.sheet' can also have the same product code appear multiple times but NOT in the inventory.sheet.

If a product code is missing in the 'inventory.sheet' can it indicate an error with a statement appearing in the missing (incoming.sheet or outgoing.sheet) ItemNum's row?

I think that covers the basic, any additional features I can't think of yet would be very welcome, thank you for your help.