I have used formula field and calculated a percentage.  I have used the output of this field to calculate one more percentage.  Now I want to add all these into another field - obviously a formula field.  However, in the refer fields block these fields are not visible.

Ex: formula field 1 = ((9/100)*CTC)

Formula field 2 = ((5/100)*formula field1)

Formula field 3 = sum of (formula field 1 + formula field 2) - however I am not able to view these options under refer field in the formula field pop up box.  

How to address this?