Let $f(x)$ be a function defined as:
$$f(x) = \begin{cases} \frac{\sqrt{1 + px} - \sqrt{1 - px}}{x}, & \text{if } -1 \leq x < 0 \\ \frac{2x + 1}{x - 2}, & \text{if } 0 \leq x \leq 1 \end{cases}$$
If $f(x)$ is continuous in the interval $[-1, 1]$, then $p =$