Given a $6\times 6$ square consisting of unit squares, denote its rows and columns from $1$ to $6$. Figure p-horse can move from square $(x; y)$ to $(x’; y’)$ if and only if both $x + x’$ and $y + y’$ are primes. At the start the p-horse is located in one of the unit squares. $a)$ Can the p-horse visit every unit square exactly once? $b$) Can the p-horse visit every unit square exactly once and with the last move return to the initial starting position?