In every row, in every column and in every area, each number from 0 to 8 occurs exactly once. The boundaries of the areas for the irregular sudoku need to be located. If there is a circle in a cell, the number in it indicates how many of the neighbouring edges and corners are used for the area boundaries (the outer frame is counted for these purposes). So, for example, the circle in r4c9 MUST contain at least a 3 because the outer frame on its right involves two corners of r4c9's cell and one line segment joining these corners).