Contents 3 5 7 9 10 11 13 15 16 17 19 20 21 1 2
2 1 1 2 A B A B A B 3 4
1 A 2 5 6
1 B 2 7 8
2 9 10
11 12
13 14
15 16
17 18
19 20
21 22