Get Encoded Name (Lex-Smallest Palindrome)

You are given a string letters that contains only lowercase letters and is itself a palindrome. Rearrange its characters and, among all rearrangements that are still palindromes, output the lexicographically smallest one.

输入格式

A single line containing the string letters. 1≤|letters|≤3×10^7; it contains only lowercase letters a-z, and letters is a palindrome.

输出格式

Output one line: the lexicographically smallest palindromic rearrangement.

样例 1

输入：

yxxy

输出：

xyyx

样例 2

输入：

cbabc

输出：

bcacb

样例 3

输入：

baab

输出：

abba

样例说明

Example 1: the characters of yxxy are two x and two y; the palindromic rearrangements are xyyx and yxxy, and the smaller one is xyyx. Example 2: in cbabc, a appears once and can only go in the center; the first half takes b and c, giving bcacb. Example 3: baab is rearranged into abba.