If formula to populate field with another field's value

If formula to populate field with another field's value

I have the following  fields in one of my queues/tabs

Claimed - currency field
Claim Pending - currency field
Credit Memo - single line field

I'd like to write the following formula.

If Credit Memo field is EMPTY than Claim Pending = Claimed
If Credit Memo field is NOT EMPTY than Claim Pending = Zero

I would appreciate any help you could provide. 

If it's helpful the name of the queue/tab is Service Request. 

Thank you,

Molly