1, in case the mouse strays off the top
1, A
4, B
4, C
8, D
9, E
9, F
11, G
11, H
12, IJ
13, K
13, L
14, M
15, N
16, 0
16, PQ
17, R
18, S
21, T
23, U
23, V
23, W
24, X
24, YZ
24, in case the mouse strays off the bottom