You can use a cross-object formula.  It would look something like:

Decision__r.Decision_Number__c

Here's the info from online help that explains when you can use a cross-object formula and how to create it.

Cross-object formulas can reference merge fields from a master ("parent") object if an object is

on the detail side of a master-detail relationship, or from another object if a lookup

relationship to that object exists on the current object. You can reference fields from objects

that are up to five relationships away. Cross-object formulas are available anywhere formulas are

used except when creating default values. To create a cross-object formula when building a formula in the Simple Formula tab, enter the

names of the objects to which you are spanning followed by the field you want to reference.

Separate the names of each object and the field with periods. For example, enter

Contact.Account.Name to reference the Account Name for a contact associated with a case in a

formula field on the Case object. Be sure to use the names of the objects and fields, not the

labels. Also, when referencing a custom object, add two underscores and the letter r to its name.

For example, Position__r.title__c references the Job Title field (title__c) on a Position custom

object. To create a cross-object formula when building a formula in the Advanced Formula tab or for

approvals or rules, such as workflow, validation, assignment, auto-response, or escalation rules,

click Insert, then click the related object to list its fields. Related objects are denoted by

a ">" sign.