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