Fill the grid with the digits 1 to 9 such that every row, column and region contains each digit exactly once. The regions are sets of nine orthogonally connected cells and have to be determined by the solver. Draw a loop that travels horizontally and vertically from cell to cell. This loop enters and exits every region exactly once. A small clue in the top left corner of a cell indicates the sum of the digits in all cells visited by the loop in that cell's region.