Rhystic Study Session

When Rhystic Study Session enters the battlefield,
put one progress counter on it.
At your end step, you may pay {x}, where {x} is
equal to the progress counters on this card. Then,
add one progress counter. Until your next turn,
when an opponent plays a spell, they may pay mana
equal to what you paid. If they don't, draw a card.

"Do you pay the one? Wait, do you pay the two?"