Tatami are rectangular mats, used to completely cover the floor of a room, without overlap.
Assuming that the only type of available tatami has dimensions 1×2, there are obviously some limitations for the shape and size of the rooms that can be covered.
For this problem, we consider only rectangular rooms with integer dimensions $a$, $b$ and even size $s = a \times b$. We use the term ‘size’ to denote the floor surface area of the room, and — without loss of generality — we add the condition $a ≤ b$.
There is one rule to follow when laying out tatami: there must be no points where corners of four different mats meet. For example, consider the two arrangements below for a 4×4 room:
The arrangement on the left is acceptable, whereas the one on the right is not: a red “X” in the middle, marks the point where four tatami meet.
Because of this rule, certain even-sized rooms cannot be covered with tatami: we call them tatami-free rooms. Further, we define $T(s)$ as the number of tatami-free rooms of size $s$.
The smallest tatami-free room has size $s = 70$ and dimensions 7×10. All the other rooms of size $s = 70$ can be covered with tatami; they are: 1×70, 2×35 and 5×14. Hence, $T(70) = 1$.
Similarly, we can verify that $T(1320) = 5$ because there are exactly 5 tatami-free rooms of size $s = 1320$: 20×66, 22×60, 24×55, 30×44 and 33×40. In fact, $s = 1320$ is the smallest room-size $s$ for which $T(s) = 5$.
Find the smallest room-size $s$ for which $T(s) = 200$.