7

1 0 1 0 1 1 1

0 1 0 1 0 1 1

1 0 1 0 1 1 1

0 1 0 1 0 1 0

1 0 1 0 1 1 1

1 0 1 0 1 1 1

1 0 1 0 1 1 1