SQL & PL/SQL :: Create Update Trigger That Allows Any Quantity Reduction Of 5 Units

i am trying to create a update trigger that allows any quantity reduction of 5 units and if the reduction is more than 5 units it blocks it and just reduces it by 5.here are two scenarios

SQL Update statement where the new Quantity value being smaller than 5 is allowed by the trigger.

-- old qty 20
update purchase_order
set quantity = 18
where po_no = 'PO11';
-- new qty 18

SQL Update statement where the new Quantity value being larger than 5 is only reduced by 5 by the trigger.

--old qty 25
update purchase_order
set quantity = 19
where po_no = 'PO15';
-- new qty 20

i just have the basic trigger code, but i think it is the math that i am not getting (i was never good at math)

create or replace trigger purchase_quantity_updt
before update of quantity on purchase_order
for each row
when (new.quantity < old.quantity )
begin
-- not sure what to put here
end;