\documentclass{article}
\usepackage{algorithmic}

\begin{document}

\begin{algorithmic}
%EndExpansion
\REQUIRE$n\geq 0$
\ENSURE$y=x^{n}$
\STATE$y\Leftarrow 1$
\STATE$X\Leftarrow x$
\STATE$N\Leftarrow n$
\WHILE{$N \neq 0$}
 \IF{$N$ is even}
  \STATE$X\Leftarrow X\times X$
  \STATE$N\Leftarrow N/2$
 \ELSE[$N$ is odd]
  \STATE$y\Leftarrow y\times X$
  \STATE$N\Leftarrow N-1$
 \ENDIF
\ENDWHILE
\end{algorithmic}%
%EndExpansion

\end{document}