The ratio of the number of marbles Kaden, Adam, and John gave away was 2:4:9. The number of marbles Kaden have away was 2 fewer than the number of marbles he had left. Adam had 1/2 as many marbles left as John. Kaden had 1/2 as many marbles left as Adam. They had a total of 56 marbles left. How many marbles did John give away?