Shingoki (from Japanese, literally "traffic lights") is a rectangular or square grid with circles in some cells. The goal is to draw a single continuous non-intersecting loop that passes through all circles. Black circles must be turned upon, white circles must be traveled straight through. In a cell with a gray circle the loop may turn or not. If a circle contains a number, this number indicates the total length of the straight lines going out of the circle.
Cross+A can solve puzzles from 3 x 3 to 30 x 30.