Rule 107.1b

Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles, triples, or sets to a specific value a player’s life total or the power and/or toughness of a creature or creature card. Example: If a 3/4 creature gets -5/-0, it’s a -2/4 creature. It doesn’t assign damage in combat. Its total power and toughness is 2. Giving it +3/+0 would raise its power to 1. Example: Viridian Joiner is a 1/2 creature with the ability “{T}: Add an amount of {G} equal to this creature’s power.” An effect gives it -2/-0, then its ability is activated. The ability adds no mana to your mana pool. Example: Chameleon Colossus is a 4/4 creature with the ability “{2}{G}{G}: This creature gets +X/+X until end of turn, where X is its power.” An effect gives it -6/-0, then its ability is activated. It remains a -2/4 creature. It doesn’t become -4/2.