Thank you @LITUATUI for the help,In Portugal the tretinoin cream costs around 4 euros for a 30 g tube. Online is much more expensive, 2 tubes cost around 23 euros.
But look for Azelac lotion, the 100 mL bottle is quite affordable and if you use 1 mL every night it will last 100 days.
Anyway, the H&S anti-dandruff shampoo for men is very cheap and you can buy it at your local supermarket. It's the most important thing to add to your regimen.
I will follow your regimen asap However, I still don't know how I should apply the tretinoin on the scalp, since I need to apply on the whole scalp.