Rule 106.8

If an effect would add mana represented by a hybrid mana symbol to a player’s mana pool, that player chooses one half of that symbol. If a colored half is chosen, one mana of that color is added to that player’s mana pool. If a generic half is chosen, an amount of colorless mana represented by that half’s number is added to that player’s mana pool.

Find this number in the current Rules