Offset faces in one go

 From:  Jfs (PAQUICINNO)
10376.1 
Hi, is there some easy way to way to offset in one click all the 4 faces in red by a number ? Here all the faces are reduce of 15 mm. I need that to easily put some adjustment for doors, for example. I can do that the hard way, of course, but if i have a lot of objects, (not always doors, and not always rectangles ) , it is quit time consuming.
Thank you.
Paquicinno
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