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.

Char-Rumbler. Yes, Char-Rumbler’s printed power is -1. While its power is -1 or 0, it simply deals no combat damage.

 

If a calculation or comparison needs to use a negative value, it does so.

See first example below.

If a calculation that would determine the result of an effect yields a negative number, zero is used instead,

See second and third examples below.

unless that effect doubles or sets to a specific value a player's life total

Beacon of Immortality. If you double a negative life total, you do the real math. A life total of -10 becomes -20.

or a creature's power and/or toughness.

Grunn, the Lonely King. If a creature’s power is less than 0 when it’s doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. For example, if an effect has given Grunn -7/-0 so that it’s a -2/5 creature, doubling its power and toughness gives it -2/+5, and it’s a -4/10 until end of turn.

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 Viridian Joiner'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}: Chameleon Colossus 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.