This is an example Odd-Even Sudoku starting grid. We can fill in the two highlighted cells. Let's
look at the top cell first. We can eliminate 1, 2, 5, 6, 7, 8, and 9 using the standard rules of
Sudoku, leaving us with 3 and 4. This cell is shaded grey, so this cell must be even, which
leaves us with just 4.
For the center cell, this cell isn't shaded which means this cell must be odd. We can eliminate
1, 2, 3, 6, 7, and 9, this leaves us with 4, 5, and 8. The only odd number remaining is 5, so this
cell must be a '5'.