Normal sudoku rules apply. The grid is covered by fog and correctly placing digits may reveal additional digits. Each digit satisfies only one of the following conditions: a) It is exactly one chess knight's move away from at least one cell containing the same digit; b) It is in its own 3x3 box (3x3 boxes are arranged in normal reading order. So eg 9 in box 9 meets this condition). c) It is the digit 5. There is one exception: the digit in the very centre of the grid satisfies exactly two conditions. No guessing is required or allowed.