Karn, Legacy Reforged

Karn, Legacy Reforged's power and toughness
are each equal to the greatest mana value among
artifacts you control.
At the beginning of your upkeep, add {c} for
each artifact you control. This mana can't be
spent to cast nonartifact spells. Until end of
turn, you don't lose this mana as steps and
phases end.