Apply coupon just the first item.

Hello. After using the “bcx” coupon code, every category (only just one not) make 50% discount. I want to make that gives customer a 50% discount on the first of each item on the cart. For example, customer buys 2 pcs of product A and 1 pcs of product B. The customer should get first item of product A and the product B the 50% discount. And important, that NOT using coupon code for one specific category). So it doesn’t matter how many pcs of each items you buy, the first one is always on 50% discount on each individual items. How can I solve this?