\documentclass[a4paper,11pt]{article}
%Use article for short documents.
\usepackage[T1]{fontenc}
\usepackage{parskip}
\usepackage{latexsym,amsmath,amssymb}
\usepackage{natbib}
\usepackage{times}

\begin{document}

\section{Introduction}

\section{Main body}

\section{Conclusion}

\end{document}
