time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

Alice bought a Congo Prime Video subscription and was watching a documentary on the archaeological findings from Factor’s Island on Loch Katrine in Scotland. The archaeologists found a book whose age and origin are unknown. Perhaps Alice can make some sense of it?

The book contains a single string of characters “a“, “b” and “c“. It has been pointed out that no two consecutive characters are the same. It has also been conjectured that the string contains an unusually long subsequence that reads the same from both sides.

Help Alice verify this by finding such subsequence that contains at least half of the characters of the original string, rounded down. Note that you don’t have to maximise the length of it.

A string a is a subsequence of a string b if a can be obtained from b by deletion of several (possibly, zero or all) characters.

Input

The input consists of a single string s (2|s|10^6 2≤|s|≤10^6). The string s consists only of characters “a“, “b“, “c“. It is guaranteed that no two consecutive characters are equal.

Output

Output a palindrome t that is a subsequence of s and |t||s|/2.

If there are multiple solutions, you may print any of them. You don’t have to maximise the length of t.

If there are no solutions, output a string “IMPOSSIBLE” (quotes for clarity).

Examples

Input

Output

Input

Output

Input

Output

Note

In the first example, other valid answers include “cacac“, “caac“, “aca” and “ccc“.


一道标准的思维题,这个题目的要求十分简单,找到一个子序列满足这个子序列是回文并且长度超过原字符串长度的一半(向下取整)

首先注意到总共只有三个字符,那么两个两个字作为一个pair来看的话,两个pair中必有一个单词重复。

由于要找的是子序列,所以我们可以直接比较头和尾的两个字符,用两个指针同时进行操作,由于一次操作四个字符,然后最后结果字符串长度+2,所以最后长度刚好等于一半

但是不能被4整除的时候似乎少了一位,这时候势必会出现多一位的情况,随便取一位作为最中间的值(此字符对回文串不构成影响)

最后发现所有均满足题意,也就不存在“Impossible”的情况咯

下面贴出代码


2019-07-21  10:56:05  Author: WindCry1





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