Author: Andrey Bogdanov
Fill in some cell with digits from 1 to 4. Each row and column should contain two empty cells and every digit exactly once. The numbers outside of the grid indicate the sum of the numbers between the two empty cells in that particular row or column.
Grid control: Click at a cell reveals the value chooser. Click at the selected value changes the state of the cell. More details about grid control are at the help page.