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Counter target spell. Clash with an opponent. If you win, at the beginning of your next main phase, you may add an amount of {C} equal to that spell's mana value. (Each clashing player reveals the top card of their library, then puts that card on the top or bottom. A player wins if their card had a higher mana value.)
You can’t add just some of the mana to your mana pool. You either add all the mana to your mana pool or none of it.
You decide whether or not to add that much {C} to your mana pool as the delayed triggered ability resolves at the beginning of your next main phase. You don’t decide before then.
