Author: Andrey Bogdanov
Shade some cells black to create a snake of length 45 - a chain of blackened squares which doesn't touch itself even at a point. It should go straight through the cells with white circles and it should make turns in the cells with black circles. The begin and the end of the snake are not given and are subjects to determine.
Grid control: Click the mouse button at the selected cell to change the color of the cell. Second click will put auxiliary cross sign, which doesn't affect correctness of the solution, but helps visually. You can also drag the content of one cell to another with the mouse. More details about grid control are at the help page.