Let the point \( (p, \, p + 1) \) lie inside the region \( E = \{ (x, y) : 3 - x \le y \le \sqrt{9 - x^2}, \, 0 \le x \le 3 \} \). If the set of all values of \( p \) is the interval \( (a, b) \), then \( b^2 + b - a^2 \) is equal to ______________.
