11 20
0 10

 0  1 -1.0
 0  2  1.0
 0  3 -1.0
 1  4 -1.0
 1  5  1.0
 2  4  1.0
 2  5  1.0
 2  6  1.0
 3  5  1.0
 3  6 -1.0
 4  7 -1.0
 4  8  1.0
 5  7  1.0
 5  8  1.0
 5  9  1.0
 6  8  1.0
 6  9 -1.0
 7 10 -1.0
 8 10  1.0
 9 10 -1.0