Discount off Single Item of Choice

I'm trying to give a discount of 50% off a single item of the customer's choice, but can't seem to do it unless it's a Buy X get Y type of thing. I would like them to get the discount even if there's only a single product in the cart, but not for more than one product per order.
How can I do that?

Hello,

Use Value Definition, set it up as:

qty discount
1 50%
2 0