CF202A.LLPS

This problem's actual name, "Lexicographically Largest Palindromic Subsequence" is too long to fit into the page headline.

You are given string s consisting of lowercase English letters only. Find its lexicographically largest palindromic subsequence.

We'll call a non-empty string s[_p_1_p_2... p__k] = _s__p_1_s__p_2... s__p__k (1  ≤  _p_1 < _p_2 < ... < p__k  ≤  |s|) a subsequence of string s = _s_1_s_2... s|s|, where |s| is the length of string s. For example, strings "abcb", "b" and "abacaba" are subsequences of string "abacaba".

String x = _x_1_x_2... x|x| is lexicographically larger than string y = _y_1_y_2... y|y| if either |x| > |y| and _x_1 = _y_1, _x_2 = _y_2, ..., x|y| = y|y|, or there exists such number r (r < |x|, r < |y|) that _x_1 = _y_1, _x_2 = _y_2, ..., x__r = y__r and x__r  +  1 > y__r  +  1. Characters in the strings are compared according to their ASCII codes. For example, string "ranger" is lexicographically larger than string "racecar" and string "poster" is lexicographically larger than string "post".

String s = _s_1_s_2... s|s| is a palindrome if it matches string rev(s) = s|s|s|s| - 1... _s_1. In other words, a string is a palindrome if it reads the same way from left to right and from right to left. For example, palindromic strings are "racecar", "refer" and "z".

本题的实际名称“字典序最大的回文子序列”过长，无法放入页面标题栏。

给定一个仅由小写英文字母组成的字符串 $s$。请找出其字典序最大的回文子序列。

我们称一个非空字符串 $s[p_1p_2\ldots p_k] = s_{p_1}s_{p_2}\ldots s_{p_k}$（其中 $1 \leq p_1 < p_2 < \ldots < p_k \leq |s|$）为字符串 $s = s_1s_2\ldots s_{|s|}$ 的一个子序列，这里 $|s|$ 表示字符串 $s$ 的长度。例如，字符串 "abcb"、"b" 和 "abacaba" 都是字符串 "abacaba" 的子序列。

字符串 $x = x_1x_2\ldots x_{|x|}$ 在字典序上大于字符串 $y = y_1y_2\ldots y_{|y|}$，当且仅当满足以下任一条件：

• $|x| > |y|$，且 $x_1 = y_1,\, x_2 = y_2,\, \ldots,\, x_{|y|} = y_{|y|}$；
• 或存在某个整数 $r$（满足 $r < |x|$ 且 $r < |y|$），使得 $x_1 = y_1,\, x_2 = y_2,\, \ldots,\, x_r = y_r$，且 $x_{r+1} > y_{r+1}$。
  字符串中字符的比较依据其 ASCII 码值。例如，字符串 "ranger" 在字典序上大于 "racecar"，字符串 "poster" 在字典序上大于 "post"。

字符串 $s = s_1s_2\ldots s_{|s|}$ 是一个回文串，当且仅当它与它的逆序字符串 $\mathrm{rev}(s) = s_{|s|}s_{|s|-1}\ldots s_1$ 完全相同。换言之，若一个字符串从左到右读与从右到左读完全一致，则该字符串为回文串。例如，回文串有 "racecar"、"refer" 和 "z"。

The only input line contains a non-empty string s consisting of lowercase English letters only. Its length does not exceed 10.

唯一的一行输入包含一个非空字符串 $s$，该字符串仅由小写英文字母组成。其长度不超过 $10$。

Print the lexicographically largest palindromic subsequence of string s.

输出字符串 $s$ 的字典序最大的回文子序列。

Among all distinct subsequences of string "radar" the following ones are palindromes: "a", "d", "r", "aa", "rr", "ada", "rar", "rdr", "raar" and "radar". The lexicographically largest of them is "rr".

在字符串 “radar” 的所有不同子序列中，以下子序列是回文："a"、"d"、"r"、"aa"、"rr"、"ada"、"rar"、"rdr"、"raar" 和 "radar"。其中字典序最大的是 "rr"。