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#include <iostream>
#include <vector>

using namespace std;
typedef long long ll;

constexpr int mSize = 2e5 + 5;

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

    int ileA, ileB;

    string a;
    cin >> a;
    ileA = ileB = 0;

    for (char x : a)
        (x == 'a') ? ++ileA : ++ileB;

    if (ileA % 2 && ileB % 2)
    {
        cout << "-1\n";
        return 0;
    }

    int r, iter;
    r = (int)a.size() - 1;
    ll result = 0;

    for (int i = 0; i < a.size()/2; i++)
    {
        iter = r;
        while (a[i] != a[iter])
            --iter;
        
        if (i != iter)
        {
            result += r - iter;
            a[iter] = a[r];
        }
        else
        {
            result += a.size() / 2 - i;
            
            ++r;
        }
        --r;
    }

    cout << result << '\n';

    return 0;
}