Multiple if statements in a formula field

Multiple if statements in a formula field

I'm trying to figure this out with no success. Can someone chime in on this one? Thank you.

FIELDS:
QTY_Suspect is a number field
Piece_Price is a currency field
Suspect_Product_Disposition is a lookup field

  1. if (input.Suspect_Product_Disposition == "Sort at customer", Piece_Price * QTY_Suspect, 
  2. if (input.Suspect_Product_Disposition == "Return to plant", Piece_Price * QTY_Suspect, 
  3. if (input.Suspect_Product_Disposition == "Scrap at Customer", Piece_Price * QTY_Suspect, 
  4. if (input.Suspect_Product_Disposition == "Return to plant for customer accommodation w/ 15% restocking fee", Piece_Price * QTY_Suspect * 0.85))))
When saving this formula i receive the following error message.

In Criteria left expression is of type BIGINT and right expression is of type STRING and the operator == is not valid