MASYU

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