I have 2 fields I need to do the formula on

Field 1: Todays_Inventory

Field 2: Todays_Shipment

I need to do the following formula

(Todays_Inventory - Todays_Shipment) - [The entry for Todays_Inventory in the previous record]

How can this be written to look into the last record entry and pull the data number for the Todays_Inventory field?

Thanks in advance.