Greetings, I am using Salesforce professional edition to service a client for just over 5 months now. I have been requested to create a report on the total mass (tonnage) of products sold and i am having trouble doing so, Any help would be appreciated.

I have tried creating a custom field "Mass" on the product object as well as a new field on the opportunity product object for a formula to calculate the total mass, but if i put this formula "Quantity* Product2.Mass__c) it tells me that the Product2 field does not exist, when it is a standard field.