12
4 9      // vertex 0's neighbors
2 3      // vertex 1's neighbors 
4        // vertex 2's neighbors 
3 6      // vertex 3's ...
5 6      // vertex 4's ...
2 7      // vertex 5's ...
10       // vertex 6's ...
1 9 11   // vertex 7's ...
6 10     // vertex 8's ...
         // vertex 9's ... (none)
0 9      // vertex 10's ...
7        // vertex 11's