For the function with domain $\{ x \mid 1 \leqq x \leqq 81 \}$,
$$y = \left( \log _ { 3 } x \right) \left( \log _ { \frac { 1 } { 3 } } x \right) + 2 \log _ { 3 } x + 10$$
Let $M$ be the maximum value and $m$ be the minimum value. Find the value of $M + m$. [4 points]