[Free] Doors

The button flips what you see on the sign above the door. There are two images, and you need to combine them to determine the code. You can try memorizing it or writing it down.

Got the 5 levels done, thought they were cool. 35 was an interesting one.
 
So.... I'm stuck on lvl 10.
The one where all the red down arrows have to be moved down, and the white arrows have to be moved up.