2 lists: how to compare and calculate difference in item numbers

Hi,

I have a list that is updated monthly which contains two columns:
Machinery code | Number of Devices

Under devices, we list machinery codes, and how many we have for each of them under Number of Devices.

What I'm trying to achieve is to compare the difference in numbers for each machinery code between two months (substract the number for each machinery code), but I'm not sure how?

Please note, some of the machinery codes may be present in one month but not in the following month (included in attached example), so I think a lookup for each machinery code would need to be performed first?

Thank you for your help.

Steve