P6724 [COCI 2015/2016 #5] MULTIGRAM

A string formed by concatenating several identical words end to end is defined as a compound word, and the words that make up this string are called the root. You need to determine whether the given string is a compound word. If it is, you need to find the root that appears first in it. If there are multiple possible roots, output the one with the shortest length. If it is not a compound word, output `-1`. **Note: If two strings can become exactly the same after rearranging the order of letters, they are considered the same word.**

One line containing a string of lowercase letters.

If the given string is not a compound word, output `-1`. Otherwise, output the shortest root that appears first.

#### Sample Explanation ##### Sample $1$ Notice that `aa` is also a root that appears first, but `a` is the shortest. ##### Sample $2$ This string consists of only one word `ab`, so it is not a compound word. ##### Sample $3$ The string is made up of two identical words `bba` and `bab`, and the one that appears first is `bba`. #### Constraints For $100\%$ of the testdata, the length of the string does not exceed $10^5$. #### Notes **This problem is translated from [COCI2015-2016](https://hsin.hr/coci/archive/2015_2016/) [CONTEST #5](https://hsin.hr/coci/archive/2015_2016/contest5_tasks.pdf) *T2 MULTIGRAM***. Translated by ChatGPT 5