Porygon

Defender
If Porygon is untapped, you may add unspent
mana onto Porygon in the form of -1/-1 counters.
{t}: Remove all -1/-1 counters on Porygon. Add
mana to your pool equal to the number of -1/-1
counters removed. Treat this mana as mana of
any color. Unspent mana is emptied at the end
of your turn.