#include <bits/stdc++.h>
using i64 = int_fast64_t;
#define repeat(i, a, b) for(int i = (a); (i) < (b); ++(i))
#define rep(i, n) repeat(i, 0, n)

namespace {
#define __DECLARE__(C)    \
    template <typename T> \
    std::ostream &operator<<(std::ostream &, const C<T> &);

#define __DECLAREM__(C)               \
    template <typename T, typename U> \
    std::ostream &operator<<(std::ostream &, const C<T, U> &);

__DECLARE__(std::vector)
__DECLARE__(std::deque)
__DECLARE__(std::set)
__DECLARE__(std::stack)
__DECLARE__(std::queue)
__DECLARE__(std::priority_queue)
__DECLARE__(std::unordered_set)
__DECLAREM__(std::map)
__DECLAREM__(std::unordered_map)

template <typename T, typename U>
std::ostream &operator<<(std::ostream &, const std::pair<T, U> &);
template <typename... T>
std::ostream &operator<<(std::ostream &, const std::tuple<T...> &);
template <typename T, std::size_t N>
std::ostream &operator<<(std::ostream &, const std::array<T, N> &);

template <typename Tuple, std::size_t N>
struct __TuplePrinter__ {
    static void print(std::ostream &os, const Tuple &t) {
        __TuplePrinter__<Tuple, N - 1>::print(os, t);
        os << ", " << std::get<N - 1>(t);
    }
};

template <typename Tuple>
struct __TuplePrinter__<Tuple, 1> {
    static void print(std::ostream &os, const Tuple &t) { os << std::get<0>(t); }
};

template <typename... T>
std::ostream &operator<<(std::ostream &os, const std::tuple<T...> &t) {
    os << '(';
    __TuplePrinter__<decltype(t), sizeof...(T)>::print(os, t);
    os << ')';
    return os;
}

template <typename T, typename U>
std::ostream &operator<<(std::ostream &os, const std::pair<T, U> &v) {
    return os << '(' << v.first << ", " << v.second << ')';
}

#define __INNER__                            \
    os << '[';                               \
    for(auto it = begin(c); it != end(c);) { \
        os << *it;                           \
        os << (++it != end(c) ? ", " : "");  \
    }                                        \
    return os << ']';

template <typename T, std::size_t N>
std::ostream &operator<<(std::ostream &os, const std::array<T, N> &c) {
    __INNER__
}

#define __DEFINE__(C)                                           \
    template <typename T>                                       \
    std::ostream &operator<<(std::ostream &os, const C<T> &c) { \
        __INNER__                                               \
    }

#define __DEFINEM__(C)                                             \
    template <typename T, typename U>                              \
    std::ostream &operator<<(std::ostream &os, const C<T, U> &c) { \
        __INNER__                                                  \
    }

#define __DEFINEW__(C, M1, M2)                                  \
    template <typename T>                                       \
    std::ostream &operator<<(std::ostream &os, const C<T> &c) { \
        std::deque<T> v;                                        \
        for(auto d = c; !d.empty(); d.pop()) v.M1(d.M2());      \
        return os << v;                                         \
    }

__DEFINE__(std::vector)
__DEFINE__(std::deque)
__DEFINE__(std::set)
__DEFINEW__(std::stack, push_front, top)
__DEFINEW__(std::queue, push_back, front)
__DEFINEW__(std::priority_queue, push_front, top)
__DEFINE__(std::unordered_set)
__DEFINEM__(std::map)
__DEFINEM__(std::unordered_map)
}  // namespace

bool is_kaibun(std::string s) {
    rep(i, s.size()) {
        if(s[i] != s[s.size() - i - 1]) return false;
    }
    return true;
}

void solve() {
    std::string s;
    std::cin >> s;
    std::string ss;
    std::vector<int> cnt;
    cnt.push_back(0);
    rep(i, s.size()) {
        if(s[i] != 'x') {
            ss += s[i];
            cnt.push_back(0);
        } else {
            cnt.back()++;
        }
    }
    if(!is_kaibun(ss)) {
        std::cout << -1 << std::endl;
    } else {
        int ans = 0;
        rep(i, cnt.size()) {
            if(cnt[i] != cnt[cnt.size() - i - 1]) {
                int t = std::max(cnt[i], cnt[cnt.size() - i - 1]);
                ans += t - cnt[i];
                cnt[i] = t;
                ans += t - cnt[cnt.size() - i - 1];
                cnt[cnt.size() - i - 1] = t;
            }
        }
        std::cout << ans << std::endl;
    }
}

int main() {
    // std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    solve();
    return 0;
}

Measured memory_bytes: 1148