Stepping back now from the joining of these two compartments, we will see that a remainder of the cube has been blocked off by the maneuver required to link 5 and 4.
This now isolated compartment (9) can be expressed as the sum of the two surrounding it.
4 + 4 + 5 + 5 = 18 = 1 + 8 = 9
As seen above, from within compartments 5 and 4 there is no entrance to compartment 9. But its contents may be inferred from what is present in 5 and 4, and from what can be felt and heard through the walls.