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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 their choice of the top or bottom. A player wins if their card had a greater mana value.)
You can't add just some of the mana. You either add all the mana or none of it.
You decide whether or not to add that much {C} as the delayed triggered ability resolves at the beginning of your next main phase. You don't decide before then.
