Convert Line Item Term to a number

Now that we can create calculated Line Item properties, I am trying to create a property that uses the Term in the calculation.  However the Term property is a string (eg. P12M) rather than a number.  I’ve created a new “Line Item Term in Months” property but does anyone have any suggestions on how to convert the Term into a number?  I know how to do it in Excel (using the SUBSTITUTE function) but this isnt available in HubSpot calcualted properties. I tried using the String-to-number function but it doesn’t wok as there are letters in the Term property.