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#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef unsigned int ui;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<long long> vll;
const ll LINF = 1e18;
const int INF = 1e9;

ll count_moves(string &s, char c) {
    ll n_moves = 0;
    int left_i = 0;
    int right_i = s.size() - 1;

    while (left_i < right_i) {
        while (s[left_i] != c) {
            ++left_i;
        }

        while (s[right_i] != c) {
            --right_i;
        }

        // cerr << left_i << " " << right_i << '\n';
        if (left_i == right_i) {
            // cnt(c) was odd
            n_moves += abs(left_i - (static_cast<int>(s.size() / 2)));
            break;
        }

        if (left_i >= right_i) {
            break;
        }

        n_moves += abs(left_i - (static_cast<int>(s.size()) - right_i - 1));
        ++left_i;
        --right_i;
    }
    return n_moves;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    string s;
    cin >> s;

    int cnt[2] = {0, 0};
    for (size_t i = 0; i < s.size(); ++i) {
        ++cnt[s[i] - 'a'];
    }
    // cerr << cnt[0] << ' ' << cnt[1] << '\n';

    if (cnt[0] % 2 == 1 && cnt[1] % 2 == 1) {
        cout << "-1" << endl;
        return 0;
    }

    if (cnt[0] == 0 || cnt[1] == 0) {
        cout << "0" << endl;
        return 0;
    }

    // cerr << "a: " << count_moves(s, 'a') << '\n';
    // cerr << "b: " << count_moves(s, 'b') << '\n';

    cout << count_moves(s, 'a') << endl;

    return 0;
}