P9251 [PA 2022] Palindrom

**This problem is translated from [PA 2022](https://sio2.mimuw.edu.pl/c/pa-2022-1/dashboard/) Practice Round [Palindrom](https://sio2.mimuw.edu.pl/c/pa-2022-1/p/pal/).** Bytie attends a computer club, so he knows what a palindrome is. A palindrome is a word that reads the same from left to right and from right to left. For example, `oko`, `kajak`, `kobyłamamałybok`, and `ababbaba` are palindromes, but `kajaki`, `zoo`, `alamakota`, and `abaababa` are not. He quickly opened his laptop and wrote down a word consisting only of the letters `a` and `b`. However, after thinking for a while, he remembered that this word is not necessarily a palindrome, so he decided to modify it. In one second, he can choose two adjacent letters and swap their positions. Can he turn this string into a palindrome using a finite number of operations (or by doing nothing)? If yes, what is the minimum number of seconds needed? Please help him write a program to compute this minimum time.

One line containing a string, which is the word written by Bytie. The string contains only the letters `a` and `b`.

Output one integer. If it is possible to transform the string into a palindrome using a finite number of operations, output the minimum time; otherwise output $-1$.

For $100\%$ of the testdata, the following holds: The string length does not exceed $2 \times 10^5$. Translated by ChatGPT 5