[AccessD] how to do this?

Hello Group,

What is the best way to do the following routine?

I have a form with a field: [ArticleNumber]. I am scanning a barcode and the Article Number is placed in the field. When the Article number is placed this number must be processed as Follows; (in the table behind the form is also a field: [Quantity]) the value of [Quantity] must be decreased with 1. When the amount of [Quantity] reaches a certain minimum a message box must appear.

How can this been done.

Thanks

Pedro Janssen