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#include <bits/stdc++.h>
//#include "boxes.h"
//#pragma GCC optimize("O3")
//#pragma GCC optimize("Ofast,no-stack-protector")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx")
//#pragma GCC target("avx,tune=native")

using namespace std;

typedef long long ll;

typedef long double ld;

ll AR = 19, BR = 13, CR = 23, XR = 228, YR = 322, MODR = 1e9 + 993;

ll myrand(){
    ll ZR = (XR * AR + YR * BR + CR) % MODR;
    XR = YR;
    YR = ZR;
    return ZR;
}

const int mod = 1e9 + 7;

ll bpow(ll x, ll y){
    if (y == 0)
        return 1;
    if (y == 1)
        return x;
    ll ret = bpow(x, y >> 1);
    ret = (ret * ret) % mod;
    if (y & 1)
        ret = (ret * x) % mod;
    return ret;
}

ll bdiv(ll x, ll y){
    return (x * bpow(y, mod - 2)) % mod;
}

const ll llinf = 2e18 + 100;

const int maxn = 1e5 + 100, inf = 1e9 + 100, sq = 300;

string s, t;

int n;

int z[26];

int main()
{
    #ifdef ONPC
    //ifstream cin("a.in");
    //ofstream cout("a.out");
    freopen("a.in", "r", stdin);
    freopen("a.out", "w", stdout);
    #else
    //ifstream cin("a.in");
    //ofstream cout("a.out");
    //freopen("trap.in", "r", stdin);
    //freopen("trap.out", "w", stdout);
    #endif // ONPC
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin >> s;
    n = s.length();
    for (auto i : s)
    if (i != 'x')
        t.push_back(i);
    int m = t.size();
    for (int i = 0; i < m / 2; i++)
    if (t[i] != t[m - 1 - i]){
        cout << -1;
        return 0;
    }
    vector<int> g = {0};
    for (int i = 0; i < n; i++)
    if (s[i] == 'x')
        g.back()++;
    else
        g.push_back(0);
    n = g.size();
    int ans = 0;
    for (int i = 0; i < n / 2; i++)
        ans += abs(g[i] - g[n - 1 - i]);
    cout << ans;
}


Measured memory_bytes: 1228