Normal sudoku rules apply. Cells separated by a single knight's move (in chess) cannot contain the same digit. Create a one-cell wide path in the grid to form a single loop that starts and ends in the central cell. The path must orthogonally touch each grey cell at least once but cannot enter any grey cell. The path cannot touch itself orthogonally. The central cell contains the lowest digit that can appear on the path. The path must visit each of the cells with a circle in them on them in strictly ascending order along the path (a cell is "visited" the first time the path orthogonally touches the cell).