/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author aryssoncf
 */



// Taken from: https://github.com/bqi343/USACO/blob/master/Implementations/content/contest/templateShort.cpp

#include <utility>
#include <vector>
#include <random>
#include <chrono>
#include <cassert>
#include <set>
#include <queue>
#include <map>
#include <numeric>
#include <algorithm>
#include <iostream>
#include <unordered_map>
#include <bitset>
#include <iomanip>
#include <complex>
#include <stack>
#include <array>
#include <unordered_set>
#include <climits>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef double db;
typedef string str;

typedef pair<int, int> pii;

typedef vector<int> vi;
typedef vector<ull> vul;
typedef vector<ll> vl;
typedef vector<pii> vpii;

#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)

#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rsz resize

#define mp make_pair
#define pb push_back
#define f first
#define s second

const int MOD = 1e9+7; // 998244353; // = (119<<23)+1
const int MX = 2e5+5;

template<class T> bool ckmin(T& a, const T& b) {
    return a > b ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) {
    return a < b ? a = b, 1 : 0; }

mt19937 rng((uint32_t)chrono::steady_clock::now().time_since_epoch().count());


//
// Created by aryssoncf on 28/10/2019.
//

#include <type_traits>


#include <iterator>


template <typename T>
constexpr auto hasBegin(int) -> decltype(std::begin(std::declval<T>()), true) {
    return true;
}

constexpr bool hasBegin(...) {
    return false;
}

template <typename T>
using IsContainer = std::integral_constant<bool, hasBegin<T>(0)>;


#ifndef JHELPER_EXAMPLE_PROJECT_IO_HPP
#define JHELPER_EXAMPLE_PROJECT_IO_HPP

#endif //JHELPER_EXAMPLE_PROJECT_IO_HPP

template<typename T>
std::istream &operator>>(std::istream &is, std::vector<T> &vec) {
    for (T &element: vec) {
        is >> element;
    }
    return is;
}

template<typename T, typename U>
std::istream &operator>>(std::istream &is, std::pair<T, U> &p) {
    is >> p.first;
    is >> p.second;
    return is;
}

template <class X, class Y>
std::ostream & operator << (std::ostream & os, std::pair <X, Y> const& p) {
    return os << p.first << " " << p.second;
}

template <class Ch, class Tr, class Container>
std::basic_ostream <Ch, Tr> & operator << (std::basic_ostream <Ch, Tr>& os, Container const& x) {
    bool first = true;
    for(auto& y : x) {
        if (first) {
            first = false;
        } else {
            os << " ";
        }
        os << y;
    }
    return os;
}

using namespace std;

class CInsertingX {

    static const int X = 'x' - 'a';

    int solve(const str &S) {
        str T;
        trav(c, S) {
            if (c != 'x') {
                T.pb(c);
            }
        }
        str RT = T;
        reverse(all(RT));
        int n = sz(T), res = -1;
        if (RT == T) {
            int idx = 0, pl = -1, pr = sz(S);
            FOR(i, 0, sz(S)) {
                if (S[i] != 'x') {
                    if (idx == (n - 1) / 2) {
                        pl = i;
                    }
                    if (idx == n / 2) {
                        pr = i;
                    }
                    idx++;
                }
            }
            res = 0;
            for (int i = pl, j = pr; i >= 0 or j < sz(S); ) {
                if (i < 0) {
                    res++;
                    j++;
                } else if (j == sz(S)) {
                    res++;
                    i--;
                } else if (S[i] == S[j]) {
                    i--;
                    j++;
                } else if (S[i] == 'x') {
                    res++;
                    i--;
                } else {
                    res++;
                    j++;
                }
            }
        }
        return res;
    }

public:
	void solve(std::istream& in, std::ostream& out) {
        str S;
        in >> S;
        int res = solve(S);
        out << res << '\n';
	}

	int randomInt(int n) {
        uniform_int_distribution<int> uid(0, n);
        return uid(rng);
    }

	str randomString() {
        const int N = 10;
        str res(randomInt(N) + 1, ' ');
        trav(c, res) {
            c = 'a' + randomInt(25);
        }
        return res;
    }

	void setup() {
        return;
        while (true) {
            str S = randomString();
            try {
                solve(S);
            } catch (...) {
                cerr << S << endl;
                break;
            }
        }
    }
};


int main() {
    std::ios_base::sync_with_stdio(false);
	CInsertingX solver;
    solver.setup();
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	in.tie(nullptr);
    out << std::fixed;
    out.precision(20);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 3772