#include <vector>
#include <list>
#include <map>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <queue>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream>
#include <cstdio>
#include <climits>
#include <complex>
#include <cstdint>
#include <tuple>

#define M_PI       3.14159265358979323846

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
inline int readInt() { int x; scanf("%d", &x); return x; }

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;


//container util

//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()


struct Edge {
    int to, cost, nm;
    Edge(int to, int cost, int nm): to(to), cost(cost), nm(nm) {}
};

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vector<Edge>> AdjList;
AdjList graph;
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)
const int INF = 100000000;

ll GCD(ll a, ll b){
    if(a < b) swap(a, b);
    if(b == 0) return a;
    return GCD(b, a%b);
}

bool cmp(const pii& a, const pii& b){
    if(a.first == b.first){
        return a.second < b.second;
    }
    return a.first < b.first;
}

bool isPalindrome(string s) {

    bool isPalindrome = true;
    int i = 0, j = s.size() - 1;
    while(isPalindrome && i <= j) {
        isPalindrome &= s[i] == s[j];
        i++;
        j--;
    }
    return isPalindrome;
}

int main()
{
    //cin.tie(0);
    //ios::sync_with_stdio(false);
    //cout << fixed << setprecision(10);

    string s; cin >> s;

    int cnt[26]={0};
    int n = s.size();

    string t="";
    for(int i=0; i<n; ++i){
        if(s[i]!='x') t += s[i];
        cnt[s[i]-'a']++;
    }

    if(isPalindrome(t) == false){
        cout << -1 << endl;
    }else{

        int len = t.size();
        int ans = 0;
        if(len%2==0){

            string str = "";
            for(int i=0; i<len/2; ++i){
                str += t[i];
            }
            //cout << str << endl;

            int l=0;
            int idx=0;
            for(l=0; l<n; ++l){
                if(s[l] == str[idx]){
                    idx++;
                }

                if(idx == len/2) break;
            }

            int r = n-1;
            idx = 0;

            for(r = n-1; r>=0; --r){
                if(s[r] == str[idx]){
                    idx++;
                }

                if(idx == len/2) break;
            }

            while(l >= 0 || r <n){
                if(l>=0 && r < n && s[l] == s[r]){
                    l--;
                    r++;
                }else {
                    if(l>=0 && s[l]=='x'){
                        ans++;
                        l--;
                    }else if(r < n && s[r]=='x'){
                        ans++;
                        r++;
                    }
                }
            }
            

        } else{
            string str = "";
            for(int i=0; i<=len/2; ++i){
                str += t[i];
            }
            //cout << str << endl;

            int i=0;
            int idx = 0;
            for(i=0; i<n; ++i){
                if(s[i]==str[idx]){
                    idx++;
                }

                if(idx == str.size()) break;
            }
            //cout << idx << endl;

            int l = i-1, r = i+1;

            while(l >= 0 || r <n){
                if(l>=0 && r < n && s[l] == s[r]){
                    l--;
                    r++;
                }else {
                    if(l>=0 && s[l]=='x'){
                        ans++;
                        l--;
                    }else if(r < n && s[r]=='x'){
                        ans++;
                        r++;
                    }
                }
            }
        }
        cout << ans << endl;
    }
    return 0;
}

Measured memory_bytes: 640