Draw a single closed loop which runs horizontally or vertically between the centers of the cells. The loop cannot intersect or touch itself. The loop must visit all cells with circles. In a cell with a black circle the loop must turn by 90° and must travel straight through the previous and next cell. In a cell with a white circle the loop must travel straight through and must turn by 90° on the previous or the next cell.
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.