Please Help me make program

number of package Delivery cost

1 $5

2-5 $10

6-10 $15

10 or more $20

where it ask user to enter a number of package to be delivered and a coupon code coupon code should be "Halfprice" to get 50% discout on delivery . The program calculate then displays the delivery charge if user dont enter correct coupon code program doesnot display price with discount.