Shade some cells in the grid. Numbers outside show the number of cells shaded in the corresponding row or column. All nodes where shaded cells form the chess pattern are shown.
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.