How to guess that shot an even or odd number of cards

Few famous and popular focus, but you must know it. This focus is based on a specially prepared pre-pack. Cards in the deck are laid out in a specific order: red card, black card, a red card, a black card, and so on. During the execution of the focus you need to see the color of the bottom card. Place the deck on the table and ask the spectator to remove the card at the same time you say that you are by weight cards taken can specify an even number of cards in the deck or odd. Take a shot with the card. They previously were part of the upper deck and put them on a hand, as if weighing them, and quietly try to see the bottom card. When this card is the same color as the bottom card of the deck throughout, then shot an even number of cards, if not, then shot an odd number of cards.