It's a little faster to do the following, once you have the diagram. First, you note that if you move from top right to top left, the area changes from (a2+a3) to (a3+a4), for a difference of (a4-a2). Then, you note that if you move from bottom left to bottom right, the area changes from (a1+a4) to (a1+a2), for a difference of (a2-a4)--exactly the opposite of what you lost when traveling left!

So now it's really easy. Going left on top, you have 16-10 = 6. Adding it back on the bottom you have 8+6 = 14. Done!