Let \( f: \mathbb{R} \times \mathbb{R} \to \mathbb{R} \) be defined by \[ f(x, y) = 8x^2 - 2y, \text{ where } \mathbb{R} \text{ denotes the set of all real numbers.} \] If \( M \) and \( m \) denote the maximum and minimum values of \( f \), respectively, on the set \[ \{(x, y) \in \mathbb{R}^2 : x^2 + y^2 = 1\}, \] then M - m = _________ (round off to 2 decimal places