MASYU

Draw a continuous line that passes through the centre of every circle in the grid. It must run vertically or horizontally, not diagonally. On a grey circle the line must turn 90° then go straight for at least one square on either side of the grey circle (see above). The line must run straight through a red circle, but must turn 90° on one or both of the adjacent squares (see above). It cannot enter any square more than once.