Normal sudoku rules apply. Placing a correct digit in the grid will clear any fog in the surrounding cells. Digits along a blue region sum line have an equal sum within each 3x3 box the line passes through. Region sum lines do not branch. Each “quadruple clue” is a white circle containing four digits – these four digits must each appear once in the four surrounding cells. The digits in each quadruple clue’s white circle are in ascending order from left to right, top to bottom.