Discount in a category

I create a new category called DISCOUNT. I need to set a discount with coupon code for a total of 50 euros only at the products of the category DISCOUNT. How can I do it?

For example

CATEGORY DISCOUNT
PRODUCT1 TOT: 35 euros
PRODUCT2 TOT: 25 euros

CATEGORY DEFAULT
PRODUCT3: 15 euros
PRODUCT4: 22 euros

The total of my order must be (35+25-50)+15+22.
It is clear in this way?

If I create a new discount "assigned to categories" with discount amount of 50 euros and I set this discount in my category, the discount was applied to all the products, every time 50 euros.

Example:
PRODUCT1 tot: 35 euro --> with discount code 0
PRODUCT2 tot: 45 euro --> with discount code 0
PRODUCT3 tot: 60 euro --> with discount code 10

The total would must be 35+45+60-50 and not 0+0+10...