P(X=m,Y=n)
=
\left\{
\begin{array}{ll}
\displaystyle
\left(
\frac 16\right)^{n-1}\frac 16\left(\frac 26\right)^{m-n-1}\frac 46, & m>n,\\
0, & m\leq n,
\end{array}
\right.
=
\left\{
\begin{array}{ll}
\displaystyle
\frac{2^{m-n+1}}{6^m}, & m>n,\\
0, & m\leq n.
\end{array}
\right.