Sauf cas particulier (mais la situation décrite ici n'en fait pas partie), si un nombre négatif doit être utilisé pour determiner le résultat d'un effet, on prend 0 à la place. D'où la réponse du gatherer.
Citation :
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.