how to add '-' defence/attack bonus

[~DietX~]

I've seen in some maps that map makers make skills like Devotion aura and others give '-' defence bonus, meaning that instead of increasing the defence of a unit it decrease it.
Whenever i try to change something to '-' it goes to 0.00 so i would be very thankfull if you tell me how to do that

EDIT: i've read the forum rules just now, and i found an answer to my question on the spells forum, sorry for posting this thread, please remove it.

 

[Need_O2]

hold shift - double click to value you want to change
now you can enter negative number