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#include <iostream>
#include <set>
#include <map>
#include <algorithm>
#include <cassert>

using namespace std;

void rewind_last_before(set<unsigned int> &pos, unsigned k) {
    while (pos.rbegin() != pos.rend() && *pos.rbegin() > k) {
        pos.erase(prev(pos.end()));
    }
}

void rewind_first_after(set<unsigned int> &pos, unsigned p) {
    while (pos.begin() != pos.end() && *pos.begin() < p) {
        pos.erase(pos.begin());
    }
}

int main() {
    ios_base::sync_with_stdio(false);

    string word;
    cin >> word;

    map<char, set<unsigned >> pos;
    pos['a'] = set<unsigned>();
    pos['b'] = set<unsigned>();

    auto i = 0;
    for (const auto &c: word) {
        pos[c].insert(i);
        i++;
    }

    unsigned p = 0;
    unsigned k = word.length() - 1;

    long a_count = count(word.begin(), word.end(), 'a');
    long b_count = count(word.begin(), word.end(), 'b');

    if (a_count % 2 == 1 && b_count % 2 == 1) {
        cout << -1 << endl;
    } else {

        unsigned long long cnt = 0;
        while (p < k) {
            if (word[p] != word[k]) {
                rewind_first_after(pos[word[k]], p);
                rewind_last_before(pos[word[p]], k);
                auto first_pos = pos[word[k]].begin();
                auto last_pos = prev(pos[word[p]].end());
                if (*first_pos - p < k - *last_pos) {
                    pos[word[p]].erase(p);
                    pos[word[p]].insert(*first_pos);
                    pos[word[k]].erase(*first_pos);
                    word[*first_pos] = word[p];
                    word[p] = word[k];
                    cnt += *first_pos - p;
                } else {
                    pos[word[k]].erase(k);
                    pos[word[k]].insert(*last_pos);
                    pos[word[p]].erase(*last_pos);
                    word[*last_pos] = word[k];
                    word[k] = word[p];
                    cnt += k - *last_pos;
                }
            }
            p++;
            k--;
        }
        cout << cnt << endl;
    }
    return 0;
}