\begin{array}{c|ccccc}
X + Y \backslash X - Y & -2 a & -a & 0 & a & 2 a \\
\hline
-2 a & 0 & 0 & 0 & 0 & 0 \\
-a & 0 & 0.25 & 0 & 0.25 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 \\
a & 0 & 0.25 & 0 & 0.25 & 0 \\
2 a & 0 & 0 & 0 & 0 & 0 \\
\end{array}