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#include<iostream>
#include<string>
#include<vector>
#include<algorithm>

using namespace std;

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

    string s;
    cin >> s;

    int n = s.size();
    int b = 0;
    for (char c : s)
    {
        if (c == 'a') ++b;
    }

    if (b & 1 && !(n & 1))
    {
        cout << "-1";
        return 0;
    }

    char c = 'a';
    if (b & 1)
    {
        c = 'b';
        b = n - b;
    }

    int left = 0;
    int right = n - 1;

    long long res = 0;
    for (int i = 0; 2 * i < b; ++i)
    {
        while (s[left] != c) ++left;
        while (s[right] != c) --right;

        res += abs(left + right - n + 1);
        ++left;
        --right;
    }

    cout << res;
}