2
5
6
1
7
1
7
5
8
9
8
7
2
1
2
7
8
9
8
5
4
8
6
2
9
8
6
5
3
1
1
8
4
1
9
2
"Chess demands total concentration." — Bobby Fischer
"Chess demands total concentration." — Bobby Fischer
When a box has only two empty cells, you know exactly which two digits fill them.
When a box has only two empty cells, you know exactly which two digits fill them.