\begin{align}
A(t) &= \frac{X_{\textrm{max}}(t) - X_{\textrm{min}}(t)}{2}, \text{where},\\
X_{\textrm{max}}(t) &= \max_{y}[x \mid \phi(x, y, t) = 0], \\
X_{\textrm{min}}(t) &= \min_{y}[x \mid \phi(x, y, t) = 0].
\end{align}
