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