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>

using namespace std;
#define all(a)a.begin(),a.end()
#define rep(i,n) for (int i = 0; i < (n); ++i)
using ll=long long;
using ll=long long;
const int INF = 1<<30;
const ll INFll =1LL<<62;
const int mod = int(1e9)+7;
using V = vector<ll>;
using P = pair<ll,ll>;

int main()
{
   string s;cin >>s;
   ll l=0,r=s.size()-1;
   ll ans=0;
   while(1){
    if(l>=r)break;
    if(s[l]==s[r]){
            l++;r--;
    }
    else if(s[l]=='x'){
        ans++;
        l++;
    }
    else if(s[r]=='x'){
        ans++;
        r--;
    }
    else{
        ans=-1;
        break;
    }
   }
   cout << ans << endl;
    return 0;
}