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#include <iostream>
#include <vector>

int64_t cost(int64_t l, int64_t r, int64_t m) {
    int64_t dist = std::max(std::abs(l-m), std::abs(r-m));
    int64_t lpos = m-dist;
    int64_t rpos = m+dist;
    return std::abs(l-lpos) + std::abs(r-rpos);
}

int64_t cost2(int64_t l, int64_t r, int64_t m) {
    int64_t dist = std::max(std::abs(l-m), std::abs(r-m-1));
    int64_t lpos = m-dist;
    int64_t rpos = m+1+dist;
    return std::abs(l-lpos) + std::abs(r-rpos);
}

int64_t solve(const std::string& p, char b) {
    std::vector<int64_t> pos;
    int64_t i = 0;
    for (char c : p) {
        if (c == b) {
            ++i;
        } else {
            pos.push_back(i);
        }
    }
    if (i%2 == 0) {
        int64_t sum = 0;
        int64_t l = 0, r = pos.size()-1;
        int64_t m = i/2;
        while (l < r) {
            sum += cost(pos[l], pos[r], m);
            ++l;
            --r;
        }
        if (l==r) sum += std::abs(pos[l]-m);
        return sum;
    } else {
        if (pos.size() %2 == 1) return -1;
        int64_t sum = 0;
        int64_t l = 0, r = pos.size()-1;
        int64_t m = i/2;
        while (l < r) {
            sum += cost2(pos[l], pos[r], m);
            ++l;
            --r;
        }
        return sum;
    }
    return 10*(i*pos.size());
}

int main() {
    std::ios_base::sync_with_stdio(0);
    std::string p;
    std::cin >> p;
    std::cout << std::min(solve(p,'a'), solve(p,'b')) << std::endl;
    return 0;
}