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#include <bits/stdc++.h>
// #define MULTIPLE_TESTS
// #define ENDLESS_TESTS
#define TIME_LIMIT 2
#define MEMORY_LIMIT 512

using namespace std;

void test()
{
    std::string text;
    text.reserve(200'001);

    std::cin >> text;

    std::vector<int> bs;
    bs.reserve(200'001);

    const int length = text.size();

    for (int i = 0; i < length; ++i)
        if (text[i] == 'b')
            bs.push_back(i);

    if (length % 2 == 0 && bs.size()%2 == 1)
    {
        std::cout << "-1\n";
        return;
    }

    long long answer = 0;

    const int pairs = bs.size() / 2;

    auto it_front = bs.begin();
    auto it_back = bs.rbegin();

    for (int i = 0; i < pairs; ++i)
    {
        const auto lhs = *(it_front++);
        const auto rhs = length - *(it_back++) - 1;

        answer += std::abs(lhs - rhs);
    }

    if (bs.size() % 2 == 1)
    {
        const auto val = *it_front;
        const auto mid = length / 2;

        answer += std::abs(val - mid);
    }

    std::cout << answer << '\n';
}


int main()
{
#ifndef CONTEST_WORKSPACE
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
#endif

#ifdef ENDLESS_TESTS
    while(!(cin >> std::ws).eof())
        test();
#else
    int T = 0;

#ifdef MULTIPLE_TESTS
    cin >> T;
#else
    T = 1;
#endif

    while (T --> 0)
        test();
#endif

    return 0;
}