Hydra of the bogs

Hydra of the bogs costs {0} to cast as long as
you control at least one hydra
{b/g}{b/g} - bestow cost
Hydra of the bogs gets +X/+X where X is
the number of all things in graveyards times ten
enchanted creature gets +X/+X where X is
the number of all things in graveyards times ten