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/*
    Zadanie: Palindrom
    Autor: Tomasz Kwiatkowski
*/

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

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    string s;
    cin >> s;
    vector<int> bs;
    int n = s.size();
    for (int i = 0; i < n; ++i) {
        if (s[i] == 'b')
            bs.push_back(i);
    }
    int m = bs.size();

    if ((m % 2 == 1) && (n % 2 == 0)) {
        cout << "-1\n";
        return 0;
    }
    long long res = 0;
    for (int i = 0; i < m / 2; ++i) {
        res += abs(bs[i] - (n - bs[m - i - 1] - 1));
    }
    if (m % 2 == 1)
        res += abs(n / 2 - bs[m / 2]);
    cout << res << '\n';
    return 0;
}