A carpenter working on a very tall house stood on the middle step of a long ladder. To install a window, he went up 4 steps. To get a tool, he went down 6 steps. Then he went up 7 steps to work on the window again. Then he went up 7 more steps to work on the next level. Finally, he went up the 7 remaining steps to the top of the ladder, and stood on the roof of the house. How many steps does the ladder have?