18
0 0 7 0
9 0 8 0
0 1 1 3
0 2 2 0
0 3 1 2
0 4 0 3
9 4 8 4
3 2 2 3
7 0 4 2
7 0 5 3
9 2 7 4
2 0 4 3
9 1 8 3
4 3 6 6
7 4 9 2
8 4 9 4
6 6 9 4
9 8 8 8