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