Brain Explosion

Brain Explosion does X damage where X is the
sum of friendly creatures power and allied players
lifepoints minus the sum of enemy creatures
toughness and enemy players lifepoints divided
by Y where Y is the number of enemy creatures
and players, rounded up, to each of Z different
targets of your choice where Z is equal to the
number of your permanents divided by the
number of enemy permanents, rounded up.
This spell costs {1} less to cast for each card any
player holds in his hand. If X would be