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: Accepted)

#include <bits/stdc++.h>
#define for0(i, n) for(int i = 0; i < n; i++)
#define for1(i, n) for(int i = 1; i <= n; i++)
#define pb push_back
#define mp make_pair
#define all(v) v.begin(), v.end()
#define V vector<int>
#define VP vector<pair<int, int> >
#define FASTIO ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
using namespace std;
#ifdef _WIN32
#include <windows.h>
#define print(x) PRINT(x, #x)
template<typename T> inline const void PRINT(T VARIABLE, string NAME)
{
#ifndef ONLINE_JUDGE /// ONLINE_JUDGE IS DEFINED ON CODEFORCES
    HANDLE hConsole = GetStdHandle(STD_OUTPUT_HANDLE);
    SetConsoleTextAttribute(hConsole, 10);
    cerr << NAME << " = " << VARIABLE;
    SetConsoleTextAttribute(hConsole, 7);
    cerr << '\n';
#endif
}
#else
#define print(x) 0
#endif
struct pair_hash
{
    template<typename T1, typename T2>
    size_t operator () (const pair<T1, T2> &p) const
    {
        auto h1 = hash<T1> {}(p.first);
        auto h2 = hash<T2> {}(p.second);
        return h1 ^ h2;
    }
};
typedef long long ll;
typedef unsigned long long ull;
const ll INFLL = 2 * (ll)1e18 + 100;
const int INFINT = 2 * (int)1e9 + 100;
//ifstream fin(".in");
//ofstream fout(".out");
void die()
{
    cout << "-1\n";
    exit(0);
}
const int NMAX = 3 * 1e5 + 5;
const int MOD = 1e9 + 7; /// careful here (7 or 9, 66.. etc)
const double PI = atan(1) * 4;
const double EPS = 1e-12;
const int seed = 1e3 + 7;

string s;
int sol;
int l, r;
int main()
{
    FASTIO;
    cin >> s;
    r = s.size() - 1;
    while(l < r)
    {
        if(s[l] == s[r])
        {
            l++;
            r--;
            continue;
        }
        sol++;
        if(s[l] == 'x')
        {
            l++;
            continue;
        }
        if(s[r] == 'x')
        {
            r--;
            continue;
        }
        return cout << -1, 0;
    }
    cout << sol;
    return 0;
}