Even & odd
Imagine a 16*16 field, whose cells are painted black and white one by one, like the chessboard. So, when our tank makes one move, the color under it changes. That is, if we put a flag on a grid which has the same color as your tank’s original position, your moves to the flag must be an even number. On the other hand, If Flag and your tank stand at grids with different colors, you will get a odd number.