Erasing Substrings

给你一个字符集是小写字母长度是$n$ 的字符串 令$k=\lfloor\log_2n\rfloor$ ,你可以依次进行$k$ 次操作 第$i$ 操作删除当前串$s$ 中一个长度是$2^{i-1}$ 的子串 你需要使得最终的字符串字典序最小,请输出这个最小的串 感谢@Kelin 提供的翻译

You are given a string $ s $ , initially consisting of $ n $ lowercase Latin letters. After that, you perform $ k $ operations with it, where . During $ i $ -th operation you must erase some substring of length exactly $ 2^{i-1} $ from $ s $ . Print the lexicographically minimal string you may obtain after performing $ k $ such operations.

The only line contains one string $ s $ consisting of $ n $ lowercase Latin letters ( $ 1<=n<=5000 $ ).

Print the lexicographically minimal string you may obtain after performing $ k $ operations.

adcbca

aba

abacabadabacaba

aabacaba

Possible operations in examples: 1. adcbca  adcba  aba; 2. abacabadabacaba  abcabadabacaba  aabadabacaba  aabacaba.