#include <bits/stdc++.h>

int main(void)
{
    std::string input;
    std::cin >> input;

    int a, b;
    long long output = 0;
    a = std::count(input.begin(), input.end(), 'a');
    b = std::count(input.begin(), input.end(), 'b');

    if (a % 2 == 1 && b % 2 == 1)
        output = -1;
    else {
        int n = input.length();
        std::vector<int> a_pos;
        for (int i = 0; i < n; ++i)
            if (input[i] == 'a')
                a_pos.push_back(i);
        
        for (int i = 0; i < a_pos.size() / 2; ++i)
            output += abs(n - a_pos[i] - a_pos[a_pos.size() - i - 1] - 1);

        if (a_pos.size() % 2 == 1)
            output += abs(a_pos[a_pos.size() / 2] - n / 2);
    }

    std::cout << output;
    return 0;
}