Help with inventory tracking [Closed]

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Hello,

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?