White cells belong to an even region. It must contain an even number of dots (0, 2, 4 …).

Grey cells belong to an odd region. It must contain an odd number of dots (1, 3, 5 …).

The number beside each row and below each column tells you exactly how many dots go in that line.

Tap any cell to cycle it through dot → dash → empty.

Every cell must end up with either a dot or a dash.