12
0  1.5  2.5
1  7    2.5
2  13   2.5
3  19.5 2.5
4  1.5  7.5
5  7    7.5
6  13   7.5
7  19   7.5
8  1.5  12.5
9  7    12.5
10 13   12.5
11 19.5 12.5