In each of the four stacked grids, normal 4x4 Sudoku rules apply. (Place the digits 1 to 4, once each, into every row, column and 2x2 box.) The value of a cell with a window is the sum of its digit and the digit in the cell from a different grid that can be seen through the window. Each value in a circle is the sum of the values on the attached arrow. Adjacent values along a green line differ by at least five.