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