Reporting Services :: Migrating All Crystal Reports Over To SSRS

I am in the middle of migrating all Crystal Reports over to SSRS and am having some troubles converting formulas used in Crystal Reports to functional expressions in SSRS. For example: My formula for one of the crystal reports is the following. Very simple logic but how would I build it as an expression to work in my SSRS report?

IF {CUST_ORDER_LINE.ORDER_QTY}-{CUST_ORDER_LINE.TOTAL_SHIPPED_QTY} < 0 THEN 0 ELSE {CUST_ORDER_LINE.ORDER_QTY}-{CUST_ORDER_LINE.TOTAL_SHIPPED_QTY}