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#include <bits/stdc++.h>
#define dbg(x) " [" << #x << ": " << (x) << "] "
using namespace std;
template<typename A, typename B>
ostream& operator<<(ostream& out, const pair<A,B>& p) {
    return out << "(" << p.first << ", " << p.second << ")";
}
template<typename T>
ostream& operator<<(ostream& out, const vector<T>& c) {
    out << "{";
    for(auto it = c.begin(); it != c.end(); it++) {
        if(it != c.begin()) out << ", ";
        out << *it;
    }
    return out << "}";
}
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    string s;
    cin >> s;
    int n = s.length();
    vector<set<int>> st(2);
    for(int i = 0; i < n; i++) {
        st[s[i] - 'a'].insert(i);
    }
    if((st[0].size() & 1) && (st[1].size() & 1)) {
        cout << -1 << endl;
        return 0;
    }
    long long ans = 0;
    for(int i = 0; i < n / 2; i++) {
        st[s[i] - 'a'].erase(i);
        st[s[n - 1 - i] - 'a'].erase(n - 1 - i);
        if(s[i] == s[n - 1 - i]) continue;
        int first = n - 1 - i;
        if(st[1 ^ (s[i] - 'a')].size()) first = *st[1 ^ (s[i] - 'a')].begin();
        int last = i;
        if(st[1 ^ (s[n - 1 - i] - 'a')].size()) last = *prev(st[1 ^ (s[n - 1 - i] - 'a')].end());
        if(first - i < n - 1 - i - last) {
            ans += first - i;
            st[s[first] - 'a'].erase(first);
            st[s[i] - 'a'].insert(first);
            swap(s[i], s[first]);
        } else {
            ans += n - 1 - i - last;
            st[s[last] - 'a'].erase(last);
            st[s[n - 1 - i] - 'a'].insert(last);
            swap(s[n - 1 - i], s[last]);
        }
    }
    cout << ans << endl;
    return 0;
}