Normal sudoku rules apply. Placing correct digits will remove fog from surrounding cells to reveal more clues. Somewhere in the fog are 9 marked cells - one in every row, column and 3x3 box. These cells must contain the digits 1-9 once each. For each marked cell, the sum of ALL the digits in the cells up to and including that marked cell, (top to bottom, left to right, as one might read) must always sum to a square number.