Normal sudoku rules apply. Digits separated by a yellow safety strip are in a 2:1 ratio (ie one is double the other.) Extend the 'gap' (ie the grey lines) along cell edges so that it forms a single loop, which cannot touch itself. Even and odd digits may not touch except across the gap. Draw a path from cell centre to cell centre from the blue dot to the red pin, which may move orthogonally or diagonally. It can't cross or touch the gap, except at yellow safety strips. Adjacent digits on the path differ by at least 3.