#!/usr/bin/perl -w
use strict;

my $input = <STDIN>;
chomp($input);

my $longest_palindrome = '';

# look for two occurrences of the same character back to back or with another
# character in-between them to find palindromes:
while ($input =~ /((.).?\2)/g)
{
	# the first character beyond the three or two character middle of a
	# palindrome:
	my $match_position = pos($input);

	# get the positions of the two matching characters:
	my $left_pos  = $match_position - length $1;
	my $right_pos = $match_position - 1;

	# now go looking to the left and right of each matching character
	# for more matching characters:
	while (nextCharactersMatch($input, $left_pos, $right_pos))
	{
		last if ($left_pos <= 0 or $right_pos >= length $input);

		$left_pos--;
		$right_pos++;
	}

	# extract the palindrome:
	my $offset     = $left_pos;
	my $length     = ($right_pos - $left_pos) + 1;
	my $palindrome = substr($input, $offset, $length);

	$longest_palindrome = $palindrome
		if ($length > length $longest_palindrome);

	# backtrack, to find palindromes within this palindrome:
	pos($input) -= (length($1) - 1);
}

print $longest_palindrome;





sub nextCharactersMatch
{
	my ($input, $left_pos, $right_pos) = @_;

	return 1 if (substr($input, $left_pos - 1, 1)
			eq substr($input, $right_pos + 1, 1));
}