begin{algorithm}
caption{Calculate $y = x^n$}
label{alg1}
begin{algorithmic}
REQUIRE $n geq 0 vee x neq 0$
ENSURE $y = x^n$
STATE $y leftarrow 1$
IF{$n < 0$}
STATE $X leftarrow 1 / x$
STATE $N leftarrow -n$
ELSE
STATE $X leftarrow x$
STATE $N leftarrow n$
ENDIF
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}
end{algorithm}