Draw a closed line which can't intersect itself by crossing all cells only one time.

This line must change direction when it is crossing through two cells following each other if this two cells
are different color (gray and black). The line must continue straight ahead if this two cells are same color (gray-gray or black-black).

Mark the cells which have changing directions.

Start from above, look at the marked cells from left to right, you will see a mathematical operation.

Enter the result of this mathematical operation.

Note : Operations are done from left to right. (Not with mathematical ordinal)

Example :