Inventory management help?

Hi,
Not sure which category this fits in but hopefully someone will be able to help us.
We're using excel to monitor our inventory. We all have pretty basic excel knowledge. We'd like to be able to track our stock levels with excel but currently our method is not particularly effective. We have a simple
'total (a1) = stock in (a2) - stock out (a3)' formula right now, but this require us to continuously add to the already present numbers (eg if we had previously received 50 items, and our next order was another 50, we would have to update a2 from 50 to 100) and this is getting confusing.
Is there a way to add an input cell where we could just type in how many we have received/used and have it added/subtracted from the total cell without requiring a constantly increasing count? Hopefully this makes sense and someone will be able to assist us,
Thanks

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You will need to re-enter inventory received regardless of any method you use.

Can you clarify your question / goal ?

Basically, is there a way we can sort of submit the data to the spreadsheet, rather than having to add to the previous number?
Right now, there is a bit of confusion being caused because say, someone will update the amount of stock they've added in by changing the number in the input column eg, if we had received 50 items last week, and had our input column say 50, and then we received 40 the next week, we should be updating to say 90, however people keep updating the input column to say 40, which throws off the totals.
Is there a way we can submit the data so we can just say how many we got in that week, and not have to update the count (just be able to type in 40 and have it update without keeping the data in the cell), or is this more of a PEBCAK issue (understandable with some of our staff members )?

Will this help?
See attached.