CF1146B.Hate "A"

Bob has a string $s$ consisting of lowercase English letters. He defines $s'$ to be the string after removing all "a" characters from $s$ (keeping all other characters in the same order). He then generates a new string $t$ by concatenating $s$ and $s'$. In other words, $t=s+s'$ (look at notes for an example).

You are given a string $t$. Your task is to find some $s$ that Bob could have used to generate $t$. It can be shown that if an answer exists, it will be unique.

鲍勃有一个由小写英文字母组成的字符串 $s$。他将 $s'$ 定义为从 $s$ 中删除所有字符 "a" 后得到的字符串（其余字符保持原有顺序）。然后，他通过将 $s$ 与 $s'$ 拼接来生成一个新的字符串 $t$。换言之，$t = s + s'$（参见注释中的示例）。

你被给定一个字符串 $t$。你的任务是找出某个可能的 $s$，使得鲍勃能用它生成 $t$。可以证明：若解存在，则该解唯一。

The first line of input contains a string $t$ ($1 \leq |t| \leq 10^5$) consisting of lowercase English letters.

输入的第一行包含一个字符串 $t$（$1 \leq |t| \leq 10^5$），由小写英文字母组成。

Print a string $s$ that could have generated $t$. It can be shown if an answer exists, it is unique. If no string exists, print ":(" (without double quotes, there is no space between the characters).

输出一个可能生成字符串 $t$ 的字符串 $s$。可以证明：若解存在，则该解唯一。若不存在这样的字符串，则输出 ":("（不带英文双引号，两个字符之间无空格）。

In the first example, we have $s = $ "aaaaa", and $s' = $ "".

In the second example, no such $s$ can work that will generate the given $t$.

In the third example, we have $s = $ "ababacac", and $s' = $ "bbcc", and $t = s + s' = $ "ababacacbbcc".

在第一个例子中，我们有 $s = $ "aaaaa"，且 $s' = $ ""。

在第二个例子中，不存在满足条件的 $s$ 能生成给定的 $t$。

在第三个例子中，我们有 $s = $ "ababacac"，且 $s' = $ "bbcc"，以及 $t = s + s' = $ "ababacacbbcc"。