Number

The Magic game uses only integers. You can't choose a fractional number, deal fractional damage, gain fractional life, and so on. If a spell or ability could generate a fractional number, the spell or ability will tell you whether to round up or down. See rule 104, "Numbers and Symbols." Most of the time, the Magic game uses only positive numbers. 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 that would determine the result of an effect needs to use a negative value, it does so. If such a calculation yields a negative number, zero is used instead, unless that effect sets a creature's power or toughness, changes a creature's power or toughness, or sets a player's life total.

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.