#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
/*----------------------------------ここからマクロ----------------------------------*/
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define vecin(a) rep(i,a.size())cin >> a[i]
#define overload4(_1,_2,_3,_4,name,...) name
/*#define rep1(n) for(int i=0;i<(int)n;++i)
#define rep2(i,n) for(int i=0;i<(int)n;++i)
#define rep3(i,a,b) for(int i=(int)a;i<(int)b;++i)
#define rep4(i,a,b,c) for(int i=(int)a;i<(int)b;i+=(int)c)*/
#define rep1(n) for(ll i=0;i<(ll)n;++i)
#define rep2(i,n) for(ll i=0;i<(ll)n;++i)
#define rep3(i,a,b) for(ll i=(ll)a;i<(ll)b;++i)
#define rep4(i,a,b,c) for(int i=(ll)a;i<(ll)b;i+=(ll)c)
#define rep(...) overload4(__VA_ARGS__,rep4,rep3,rep2,rep1)(__VA_ARGS__)
#ifdef _DEBUG
#define debug1(a) cerr << #a << ": " << a << "\n"
#define debug2(a,b) cerr << #a << ": " << a << ", " << #b << ": " << b << "\n"
#define debug3(a,b,c) cerr << #a << ": " << a << ", " << #b << ": " << b << ", " << #c << ": " << c << "\n"
#define debug4(a,b,c,d) cerr << #a << ": " << a << ", " << #b << ": " << b << ", " << #c << ": " << c << ", " << #d << ": " << d << "\n"
#define debug(...) overload4(__VA_ARGS__,debug4,debug3,debug2,debug1)(__VA_ARGS__)
#define vecout(a) cerr << #a << ": [";rep(i,a.size()){cerr << a[i];cerr << (i == a.size() - 1 ? "":",");}cerr << "]\n"
#else
#define debug(...)
#define vecout(a)
#endif
#define mp make_pair
//struct doset{doset(int n){cout << fixed << setprecision(n);cerr << fixed << setprecision(n);}};
//struct myset{myset(){ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);}};
void myset(){ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);}
void doset(int n){cout << fixed << setprecision(n);}
using ll = long long;
using ld = long double;
using dou = double;
const int inf = 1 << 30;
const ll INF = 1LL << 60;
const ld pi = 3.14159265358;
const ll mod1 = 1000000007LL;
const ll mod2 = 998244353LL;
typedef pair<ll,ll> P;
template<class T, class U> inline bool chmin(T& a, const U& b){ if(a > b){ a = b; return 1; } return 0; }
template<class T, class U> inline bool chmax(T& a, const U& b){ if(a < b){ a = b; return 1; } return 0; }
template<class T, class U> inline bool change(T& a,U& b){if(a > b){swap(a,b);return 1;}return 0;}
//nのm乗をMODで割ったあまりO(logm)
ll modpow(ll n,ll m,ll MOD){
    if(m == 0)return 1;
    if(m < 0)return 0;
    ll res = 1;
    n %= MOD;
    while(m){
        if(m & 1)res = (res * n) % MOD;
        m >>= 1;
        n *= n;
        n %= MOD;
    }
    return res;
}

ll mypow(ll n,ll m){
    if(m == 0)return 1;
    if(m < 0)return -1;
    ll res = 1;
    while(m){
        if(m & 1)res = (res * n);
        m >>= 1;
        n *= n;
    }
    return res;
}

//素数判定O(sqrt(N))
template<class T>
inline bool isp(T n){
    bool res = true;
    if(n == 1 || n == 0)return false;
    else{
        for(ll i = 2;i * i <= n;i++){
            if(n % i == 0){
                res = false;
                break;
            }
        }
        return res;
    }
}

template<class T = int>
T in(){T x;cin >> x;return x;}

inline bool Yes(bool b){cout << (b ? "Yes\n":"No\n");return b;}
inline bool YES(bool b){cout << (b ? "YES\n":"NO\n");return b;}

/*----------------------------------マクロここまで----------------------------------*/

ll __lcm(ll a,ll b){
    return a / __gcd(a,b) * b;
}


int main(){
    myset();
    string S;
    cin >> S;
    string H = "";
    for(char c : S){
        if(c != 'x')H += c;
    }
    string H2 = H;
    reverse(all(H2));
    if(H2 != H){
        cout << "-1\n";
        return 0;
    }
    string M = "",A = "";
    if(H.size() % 2){
        ll cnt = 0,to = H.size() / 2 + 1;
        for(char c : S){
            if(c != 'x')cnt++;
            M += c;
            if(cnt >= to)break;
        }
        cnt = 0;
        for(int i = S.size() - 1;i >= 0;i--){
            if(S[i] != 'x')cnt++;
            A += S[i];
            if(cnt >= to)break;
        }
    }
    else{
        ll cnt = 0,to = H.size() / 2;
        for(char c : S){
            if(c != 'x')cnt++;
            M += c;
            if(cnt >= to)break;
        }
        cnt = 0;
        for(int i = S.size() - 1;i >= 0;i--){
            if(S[i] != 'x')cnt++;
            A += S[i];
            if(cnt >= to)break;
        }
    }
    debug(M,A);
    ll ans = 0;
    while(M != "" || A != ""){
        if(M == ""){
            assert(A[0] == 'x');
            A.erase(A.begin());
            ans++;
        }
        else if(A == ""){
            assert(M[0] == 'x');
            M.erase(M.begin());
            ans++;
        }
        else{
            if(M[0] == A[0]){
                M.erase(M.begin());
                A.erase(A.begin());
            }
            else if(M[0] == 'x'){
                M.erase(M.begin());
                ans++;
            }
            else if(A[0] == 'x'){
                A.erase(A.begin());
                ans++;
            }
            else abort();
        }
    }
    cout << ans << "\n";
}


Measured memory_bytes: 3928