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 sets a player’s life total to a specific value, doubles a player’s life total, sets a creature’s power or toughness to a specific value, or otherwise modifies a creature’s power or toughness.
Example: If a 3/4 creature gets -5/-0, it’s a -2/4 creature. It assigns 0 damage in combat. Its total power and toughness is 2. You’d have to give it +3/+0 to raise its power to 1.
Example: Viridian Joiner is a 1/2 creature with the ability “: Add to your mana pool an amount of 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.

107.1c If a rule or ability instructs a player to choose “any number,” that player may choose any positive number or zero, unless something (such as damage or counters) is being divided or distributed among “any number” of players and/or objects. In that case, a nonzero number of players and/or objects must be chosen if possible.