Enumerate Counter Style

\documentclass{article}


\begin{document}
\section{SSE}
\subsection{SS1}
Hello, world!

\begin{enumerate}
  \item First item
  \item Second item
  \item Third item
\end{enumerate}

\subsection{SS2}
\makeatletter
\renewcommand\theenumi{\arabic{section}.\@arabic\c@enumi}
% \renewcommand\theenumii{\arabic{section}.\@arabic\c@enumii}
\renewcommand\p@enumii{\theenumi-}
\def\labelenumi{\theenumi}
\def\labelenumii{\p@enumii\Alph{enumii}}
\makeatother
HELLO, WORLD!
\begin{enumerate}
  \item First item
  \begin{enumerate}
    \item First 1 item 
    \item First 2 item
  \end{enumerate}
  \item Second item
  \item Third item
\end{enumerate}

\end{document}