How to use IsNullOrEmpty ?

Scenario: User has created additional fields in Purchase order.

review PO, user wants to display result as below since Allocation 4 is updated

If set formula like this,
Expression [Allocation_2] + [Allocation_4]

System will not display result  

Because user is just updated Allocation 4 but without Allocation 2

So how should we set the expression in this scenario?

 Solution:  

Step 1: Add Calculated Field

Step 2: Point to the field --> Edit Calculated Fields

 Step 3: Edit Name

Step 4: Update Expression as below
 Iif(IsNullOrEmpty([Allocation_1]),0 ,[Allocation_1] ) + Iif(IsNullOrEmpty([Allocation_2]),0 ,[Allocation_2] )

Tips: