help me with monthly HPR % (return). I'm confused about the opening price/closing price. Do we leave empty HPR at the beginning or at the end?

\begin{tabular}{|l|l|l|} \hline Date & Adj Close & HPR % \\ \hline 4/1/2018 & 312.46 \\ \hline 5/1/2018 & 351.60 \\ \hline 6/1/2018 & 391.43 \\ \hline 7/1/2018 & 337.45 \\ \hline 8/1/2018 & 367.68 \\ \hline 9/1/2018 & 374.13 \\ \hline \end{tabular}