Leo, Angela and Elon are playing pokemon cards. Each of them starts with a few cards in hand. After Leo gives Angela 6 cards, Leo and Angela have the same number of cards. Then Angela gives 6 cards to Elon, so that Angela and Elon have the same number of cards. Finally, Elon gives 3 cards to Leo. Now Leo has twice as many cards as Elon. How many cards does Leo have at the beginning?