#include <bits/stdc++.h>

#define PB push_back
#define MP make_pair
#define ALL(x) (x).begin(),(x).end()
using namespace std;
typedef long long ll;
///////////////////////////////////////////////
/* (๑╹◡╹) */
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
template<class T>
inline bool chmin(T &a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}
 /* (๑╹◡╹)(๑╹◡╹)(๑╹◡╹)*/
template<class T>
inline bool chmax(T &a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
}
/* (๑╹◡╹)(๑╹◡╹)(๑╹◡╹)(๑╹◡╹)(๑╹◡╹)(๑╹◡╹) */
///////////////////////////////////////////////
const int MAX = 510000;

long long fac[MAX], finv[MAX], inv[MAX];

// modint (1000000007 で割ったあまりを扱う構造体)
template<int MOD> struct Fp {
    long long val;
    constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
        if (val < 0) v += MOD;
    }
    constexpr int getmod() { return MOD; }
    constexpr Fp operator - () const noexcept {
        return val ? MOD - val : 0;
    }
    constexpr Fp operator + (const Fp& r) const noexcept { return Fp(*this) += r; }
    constexpr Fp operator - (const Fp& r) const noexcept { return Fp(*this) -= r; }
    constexpr Fp operator * (const Fp& r) const noexcept { return Fp(*this) *= r; }
    constexpr Fp operator / (const Fp& r) const noexcept { return Fp(*this) /= r; }
    constexpr Fp& operator += (const Fp& r) noexcept {
        val += r.val;
        if (val >= MOD) val -= MOD;
        return *this;
    }
    constexpr Fp& operator -= (const Fp& r) noexcept {
        val -= r.val;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr Fp& operator *= (const Fp& r) noexcept {
        val = val * r.val % MOD;
        return *this;
    }
    constexpr Fp& operator /= (const Fp& r) noexcept {
        long long a = r.val, b = MOD, u = 1, v = 0;
        while (b) {
            long long t = a / b;
            a -= t * b; swap(a, b);
            u -= t * v; swap(u, v);
        }
        val = val * u % MOD;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr bool operator == (const Fp& r) const noexcept {
        return this->val == r.val;
    }
    constexpr bool operator != (const Fp& r) const noexcept {
        return this->val != r.val;
    }
    friend constexpr ostream& operator << (ostream &os, const Fp<MOD>& x) noexcept {
        return os << x.val;
    }
    friend constexpr istream& operator >> (istream &is, Fp<MOD>& x) noexcept {
        return is >> x.val;
    }
    friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
        if (n == 0) return 1;
        auto t = modpow(a, n / 2);
        t = t * t;
        if (n & 1) t = t * a;
        return t;
    }
};

// 二項係数ライブラリ
template<class T> struct BiCoef {
    vector<T> fact_, inv_, finv_;
    constexpr BiCoef() {}
    constexpr BiCoef(int n) noexcept : fact_(n, 1), inv_(n, 1), finv_(n, 1) {
        init(n);
    }
    constexpr void init(int n) noexcept {
        fact_.assign(n, 1), inv_.assign(n, 1), finv_.assign(n, 1);
        int MOD = fact_[0].getmod();
        for(int i = 2; i < n; i++){
            fact_[i] = fact_[i-1] * i;
            inv_[i] = -inv_[MOD%i] * (MOD/i);
            finv_[i] = finv_[i-1] * inv_[i];
        }
    }
    constexpr T com(int n, int k) const noexcept {
        if (n < k || n < 0 || k < 0) return 0;
        return fact_[n] * finv_[k] * finv_[n-k];
    }
    constexpr T fact(int n) const noexcept {
        if (n < 0) return 0;
        return fact_[n];
    }
    constexpr T inv(int n) const noexcept {
        if (n < 0) return 0;
        return inv_[n];
    }
    constexpr T finv(int n) const noexcept {
        if (n < 0) return 0;
        return finv_[n];
    }
};

const int MOD = 1000000007;
using mint = Fp<MOD>; //intの代わりにmintって書くとMODで割ってくれる．


int main() {
    int ans = 0;
    string S; cin >> S;

    int l_count = 0;
    int r_count = 0;
    int r = S.size()-1;
    for (int i = 0; i < S.size(); i++)
    {
        if(S[i] == 'x'){l_count++;}
        else
        {
            for (int j = r; j >= 0; j--)
            {
                if(S[j] == 'x'){r_count++;}
                else
                {
                    if(S[i] != S[j]){ans = -1;}
                    else
                    {
                        ans += abs(l_count - r_count);
                        l_count = 0;
                        r_count = 0;
                        r = j-1;
                    }
                    break;
                }
            }
            
        }
        if(ans == -1){break;}
        if(i >= r){break;}
    }
        cout << ans << endl;
	}


Measured memory_bytes: 512