2
4
1
6
4
7
1
7
9
5
8
3
2
4
7
5
5
1
3
9
7
9
3
8
2
6
9
5
2
1
6
9
2
4
4
7
6
"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.