Normal 6x6 Sudoku rules apply. ie Place the digits 1-6, once each, in every row, column and 2x3 rectangle. Draw a path along interior gridlines from the robot to the exit. The path cannot touch itself. The robot computes the difference between each pair of digits it passes between, and writes these differences to the grey memory cells in reading order. (All memory cells must be written to and no difference goes unrecorded).