Author: Andrey Bogdanov
Draw a single continuous loop in the grid using horizontal and vertical line segments so that the loop visits every square exactly once. It should not cross or overlap itself. It goes straight at every square with a circle. There is exactly one square between two consecutive circles that the loop visits where the loop does not make any turn.
Grid control: To draw a line press the mouse button at a start cell and move mouse to another cell holding the button. More details about grid control are at the help page.