A12525. 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.

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

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).

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".