When do I announce my parry?

In Das Schwarze Auge (4.1), when do I have to announce that I want to parry my enemy’s attack? Do I have to declare that before his attack is rolled and I know if he hits me or not, or can I wait to declare it after? I can’t find the page in the rules where this is stated. My guess would be to announce it after the attack, because otherwise you would just waste your one reaction per turn very often.