\begin{equation}
    \label{eq2}
    \begin{split}
    \textrm{AL = }
    \frac{\sum_{n=1}^{N} \textrm{Message arrival time} -\textrm{Message creation time}}{\textrm{\emph{N} delivered messages}}, \text{if } N\geq 1
    \\ \emph{cannot be defined}, \text{if } N = 0
    \end{split}
\end{equation}
