Rule 1: Connected cells with the same number form a region. Each region must have exactly that many cells (a region of 1s = 1 cell, 2s = 2 cells, 3s = 3 cells).
Rule 2: Two separate regions of the same size may NOT touch each other orthogonally (up/down/left/right).
Controls: Select a value (1/2/3/✖), then click cells to place. Keys: 1/2/3 select, E erase, H hint, R clear, Enter check, Esc menu.