Build a Battleship station App

In the 'Build the Object Model' section in the Build a Battleship station App, I cannot complete the 'add total cost' field. 

1. Since we want to see the total cost for each supply record, enter the following for the formula: Quantity__c * Unit_Cost__c. 

When I input the formula, I get an error (see attached).

The second screenshot is what I get when trying to verify the steps.

If anyone has any tips that would be helpful! Thanks.