Counter target spell. At the beginning of your next precombat main phase, add X mana in any combination of colors, where X is that spell's converted mana cost.
4/15/2013 If the countered spell had in its mana cost, include the value chosen for that X when determining how much mana to add to your pool. For example, if you counter a spell that costs with X equal to 5, you’ll add six mana to your mana pool.
4/15/2013 Your precombat main phase is the main phase that happens immediately after your draw step, not any other main phase.