Normal sudoku rules apply. Draw a loop (which never intersects with itself) through the centres of some cells. The loop moves from cell to cell orthogonally. Every cell in the grid is one of three types: fully inside the loop, fully outside the loop, or part of the loop. A 2x2 area cannot be entirely one cell type. A green circle is inside the loop, a blue circle is outside the loop, and a yellow circle is part of the loop. A digit placed in a circle is the number of cells of its own type that it “sees” horizontally and vertically, including itself. (Other cell types obstruct vision.) Cells inside the loop form one orthogonally connected area and cells outside the loop are orthogonally connected to the edge of the grid. Adjacent non-loop cells contain digits that differ by 5 or more.