For real numbers $a$ and $b$, consider the function $f : \mathbb{R} \to \mathbb{R}$ given by
\[ f(x) = \begin{cases}
-ax - b ;& \text{if } x \\
5x + 1 ;& \text{if } -1 \le x \le 1, \\
a^2x + 3b ;& \text{if } x > 1 .
\end{cases} \]
How many pairs $(a, b)$ are there for which $f$ is continuous at every point of $\mathbb{R}$?