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 fi first
#define se second
#define mp make_pair
#define gc getchar
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(),(x).end()
#define x1 uvuvwevwevwe
#define y1 onyetenyevwe
#define x2 ugwemubwem
#define y2 ossas
typedef long long LL;
typedef long double LD;
typedef pair<int,int> pii;
template <typename T> void scan(T &angka){
	angka=0;char input=gc();T kali=1;
	while(!(48<=input&&input<=57)){	if(input=='-')	kali=-1;input=gc();}
	while(48<=input&&input<=57)	angka=(angka<<3)+(angka<<1)+input-48,input=gc();angka*=kali;
}
template <typename T1,typename T2> void scan(T1 &angka1,T2 &angka2){
	scan(angka1);scan(angka2);
}
int panjang;
string input;
vector <char> tanpax;
vector <int> jarak;
template <typename T> int ispalin(vector <T> daftar){
	int l,r,ret=0;
	l=((daftar.size()+1)/2)-1;
	r=l;
	if((daftar.size()&1)==0)
		r++;
	while(l>=0)
	{
		ret+=abs(daftar[l]-daftar[r]);
		l--;r++;
	}
	return ret;
}
int main()
{
	cin>>input;
	panjang=input.length();
	for(int i=0;i<panjang;i++)
	{
		if(input[i]=='x')
			continue;
		tanpax.pb(input[i]);
	}
	if(ispalin(tanpax)!=0)
	{
		printf("-1\n");
		return 0;
	}
	//buat jarax
	int temp=0;
	for(int i=0;i<panjang;i++)
	{
		if(input[i]=='x')
			temp++;
		else
		{
			jarak.pb(temp);
			temp=0;
		}
	}
	jarak.pb(temp);
	cout<<ispalin(jarak)<<endl;
}