20
AB 1 1
BC 2 1
CD 1 1
DE 2 1
EF 1 1
FG 2 1
GH 1 1
HI 2 1
IJ 1 1
JK 2 1
KL 1 1
LM 2 1
MN 1 1
NO 2 1
OP 1 1
PQ 2 1
QR 1 1
RS 2 1
ST 1 1
TU 2 1