#include <iostream>
#include <string>

char other(char c)
{
    if (c == 'a')
    {
        return 'b';
    }
    else
    {
        return 'a';
    }
}

int min_moves(std::string str)
{
    // Base case
    if (str.size() == 1)
    {
        return 0;
    }
    else if (str.size() == 2)
    {
        if (str[0] == str[1])
        {
            return 0;
        }
        else
        {
            return -1;
        }
    }

    int length = str.size();
    if (str[0] == str[length - 1])
    {
        return min_moves(str.substr(1, length - 2));
    }
    else
    {
        int total = 0;
        int left_ind = str.find(other(str[0]));
        int right_ind = str.rfind(other(str[str.size() - 1]));
        if (left_ind + 1 >= length - right_ind)
        {
            for (int i = right_ind; i < length - 1; ++i)
            {
                std::swap(str[i], str[i + 1]);
                total += 1;
            }
        }
        else
        {
            for (int i = 0; i < left_ind; ++i)
            {
                std::swap(str[i], str[i + 1]);
                total += 1;
            }
        }
        int partial = min_moves(str.substr(1, length - 2));
        if (partial == -1)
        {
            return -1;
        }
        else
        {
            return partial + total;
        }
    }
}

int main()
{
    std::string input;
    std::cin >> input;
    std::cout << min_moves(input) << "\n";
    return EXIT_SUCCESS;
}