Problem:
Score : 400 points

Problem Statement

We have a string s consisting of lowercase English letters.
Snuke can perform the following operation repeatedly:

Insert a letter x to any position in s of his choice, including the beginning and end of s.

Snuke's objective is to turn s into a palindrome.
Determine whether the objective is achievable. If it is achievable, find the minimum number of operations required.

Notes

A palindrome is a string that reads the same forward and backward.
For example, a, aa, abba and abcba are palindromes, while ab, abab and abcda are not.

Constraints

1 \leq |s| \leq 10^5

s consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

s

Output

If the objective is achievable, print the number of operations required.
If it is not, print -1 instead.

Sample Input 1

xabxa

Sample Output 1

2

One solution is as follows (newly inserted x are shown in bold):

xabxa → xaxbxa → xaxbxax

Sample Input 2

ab

Sample Output 2

-1

No sequence of operations can turn s into a palindrome.

Sample Input 3

a

Sample Output 3

0

s is a palindrome already at the beginning.

Sample Input 4

oxxx

Sample Output 4

3

One solution is as follows:

oxxx → xoxxx → xxoxxx → xxxoxxx

Solution in C++14 (GCC 5.4.1) (judge verdict: Runtime Error)

#include <bits/stdc++.h>
using namespace std;

int main() {
	string s;
	cin >> s;
	vector<int> p;
	string t = "";
	for (int i = 0; i < s.size(); i++) {
		if (s[i] != 'x') {
			t += s[i];
			p.emplace_back(i);
		}
	}
	string u = t;
	reverse(u.begin(), u.end());
	if (t != u) {
		cout << -1 << endl;
		return 0;
	}
	vector<int> a(1, 0), b(1, 0);
	for (int i = 0; i < p[p.size() / 2]; i++) {
		if (s[i] == 'x') a.back()++;
		else a.emplace_back(0);
	}
	for (int i = s.size() - 1; p[(p.size() + 1) / 2] < i; i--) {
		if (s[i] == 'x') b.back()++;
		else b.emplace_back(0);
	}
	int ans = 0;
	for (int i = 0; i < a.size(); i++) {
		ans += abs(a[i] - b[i]);
	}
	cout << ans << endl;
	return 0;
}