8
0 a 1
0 b 2
1 a 3
1 b 4
2 a 5
2 b 6
3 a 1
3 b 4
4 a 4
4 b 5
5 a 4
5 b 5
6 a 7
6 b 7
7 a 6
7 b 6
0 2
0