20 43
0 1
0 5
1 2
1 6
1 7
2 3
2 6
2 7
3 4
3 8
3 9
4 8
4 9
5 6
5 10
5 11
6 7
6 10
6 11
7 8
7 12
7 13
8 9
8 12
8 13
9 14
10 11
10 15
11 12
11 16
11 17
12 13
12 16
12 17
13 14
13 18
13 19
14 18
14 19
15 16
16 17
17 18
18 19

 0--1--2--3--4
 |  |\/|  |\/|
 |  |/\|  |/\|
 5--6--7--8--9
 |\/|  |\/|  |
 |/\|  |/\|  |
10-11-12-13-14
 |  |\/|  |\/|
 |  |/\|  |/\|
15-16-17-18-19
