#pragma GCC optimize ("O3")
#pragma GCC target ("avx")
#include "bits/stdc++.h" // define macro "/D__MAI"

using namespace std;
typedef long long int ll;

#define xprintf(fmt,...) fprintf(stderr,fmt,__VA_ARGS__)
#define debugv(v) {printf("L%d %s > ",__LINE__,#v);for(auto e:v){cout<<e<<" ";}cout<<endl;}
#define debuga(m,w) {printf("L%d %s > ",__LINE__,#m);for(int x=0;x<(w);x++){cout<<(m)[x]<<" ";}cout<<endl;}
#define debugaa(m,h,w) {printf("L%d %s >\n",__LINE__,#m);for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){cout<<(m)[y][x]<<" ";}cout<<endl;}}
#define ALL(v) (v).begin(),(v).end()
#define repeat(cnt,l) for(auto cnt=0ll;(cnt)<(l);++(cnt))
#define iterate(cnt,b,e) for(auto cnt=(b);(cnt)!=(e);++(cnt))
#define MD 1000000007ll
#define PI 3.1415926535897932384626433832795
#define EPS 1e-12
template<typename T1, typename T2> ostream& operator <<(ostream &o, const pair<T1, T2> p) { o << "(" << p.first << ":" << p.second << ")"; return o; }
template<typename iterator> inline size_t argmin(iterator begin, iterator end) { return distance(begin, min_element(begin, end)); }
template<typename iterator> inline size_t argmax(iterator begin, iterator end) { return distance(begin, max_element(begin, end)); }
template<typename T> T& maxset(T& to, const T& val) { return to = max(to, val); }
template<typename T> T& minset(T& to, const T& val) { return to = min(to, val); }

mt19937_64 randdev(8901016);
inline ll rand_range(ll l, ll h) {
    return uniform_int_distribution<ll>(l, h)(randdev);
}

#ifdef __MAI
#define getchar_unlocked getchar
#define putchar_unlocked putchar
#endif
#ifdef __VSCC
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#endif
namespace {
#define isvisiblechar(c) (0x21<=(c)&&(c)<=0x7E)
    class MaiScanner {
    public:
        template<typename T> void input_integer(T& var) {
            var = 0;
            T sign = 1;
            int cc = getchar_unlocked();
            for (; cc<'0' || '9'<cc; cc = getchar_unlocked())
                if (cc == '-') sign = -1;
            for (; '0' <= cc&&cc <= '9'; cc = getchar_unlocked())
                var = (var << 3) + (var << 1) + cc - '0';
            var = var*sign;
        }
        inline int c() { return getchar_unlocked(); }
        inline MaiScanner& operator>>(int& var) {
            input_integer<int>(var);
            return *this;
        }
        inline MaiScanner& operator>>(long long& var) {
            input_integer<long long>(var);
            return *this;
        }
        inline MaiScanner& operator>>(string& var) {
            int cc = getchar_unlocked();
            for (; !isvisiblechar(cc); cc = getchar_unlocked());
            for (; isvisiblechar(cc); cc = getchar_unlocked())
                var.push_back(cc);
            return *this;
        }
        template<typename IT> void in(IT begin, IT end) {
            for (auto it = begin; it != end; ++it) *this >> *it;
        }
    };
}
MaiScanner scanner;



// オンライン回文判定
bool iskaibun(const string& str) {
    int left = 0;
    int right = str.size() - 1;
    while (left < right) {
        if (str[left] != str[right]) return false;
        ++left; --right;
    }
    return true;
}

// 

ll m, n, kei;


// ある地点iを中心としたとき，左のx以外の文字の数と→のx以外の文字の数が一致していればおｋ

string str;
string str2;
vector<int> ptr;

int chrcnt[100010][30];

int main() {

    scanner >> str;

    n = str.size();

    repeat(i, n) {
        if (str[i] != 'x')
            str2.push_back(str[i]),
            ptr.push_back(i);
    }
    
    if (str2.size() == 0) {
        cout << 0 << endl;
        return 0;
    }

    if (!iskaibun(str2)) {
        cout << -1 << endl;
        return 0;
    }

    int left;
    int right;
    if (str2.size() % 2 == 1) {
        // 奇数！
        int center = ptr[str2.size() / 2];
        left = center;
        right = center;
    }
    else {
        left = ptr[str2.size() / 2 - 1];
        right = ptr[str2.size() / 2];
    }

    int cnt = 0;
    while (0 <= left && right < n) {
        // cout << left << " " << right << endl;
        if (str[left] == str[right] && str[left] != 'x') {
            --left; ++right;
        }
        else {
            cnt += str[left] != str[right];
            if (str[left] == 'x')
                --left;
            if (str[right] == 'x')
                ++right;
        }
    }
    while (0 <= left)
        --left, ++cnt;
    while (right < n)
        ++right, ++cnt;
    cout << cnt << endl;


    // repeat(i, n) {
    //     chrcnt[i][str[i] - 'a'] = 1;
    // }
    // 
    // repeat(i, n) {
    //     repeat(j, 28) {
    //         chrcnt[i + 1][j] += chrcnt[i][j];
    //     }
    // }


    return 0;
}

Measured memory_bytes: 1148