read n
read a
a=`echo -e "${a// /\n}" | sort -n | uniq -c`
head1=`echo "${a}" | awk 'NR==1 {print $1}'`
head2=`echo "${a}" | awk 'NR==1 {print $2}'`
end1=`echo "${a}" | awk 'END {print $1}'`
end2=`echo "${a}" | awk 'END {print $2}'`
if [ $head2 -eq 1 ]
then
	if [ $head1 -ne 1 ]
	then
		echo Impossible
		exit
	elif [ $end2 -eq 2 ]
	then
		echo Possible
		exit
	fi
fi
