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#include <bits/stdc++.h>
using namespace std;
using ll = long long;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

template<typename T, typename comp = less<T>>
using ordered_set = tree<T, null_type, comp, rb_tree_tag, tree_order_statistics_node_update>;

#ifdef DEBUG
#include "debug.h"
#else
#define debug(...) ;
#endif

#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)

#define rep(i, n) for (int i = 0; i < (n); ++i)
#define repp(i, n, m) for (int i = (n); i < (m); ++i)

#define repr(i, n) for (int i = (n) - 1; i >= 0; --i)
#define reppr(i, n, m) for (int i = (m) - 1; i >= (n); --i)

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using pi = pair<int, int>;
using pll = pair<ll, ll>;

template <typename T, typename V> void mini(T& a, V b) {if (b < a) a = b; }
template <typename T, typename V> void maxi(T& a, V b) {if (b > a) a = b; }

void solve() {
    string s;
    cin >> s;

    ordered_set<int> allpos;
    deque<int> a, b;

    for (int i = 0; i < (int)s.size(); ++i) {
        allpos.insert(i);
        if (s[i] == 'a')
            a.push_back(i);
        else
            b.push_back(i);
    }

    if (a.size() & b.size() & 1) {
        cout << "-1\n";
        return;
    }

    ll cost = 0;

    while (a.size() + b.size() > 1) {
        if (a.empty() || b.empty())
            break;

        if (a.back() < b.back())
            swap(a, b);

        if (a.size() == 1) {
            cost += b.size() / 2;
            break;
        }

        int al = a.front();
        a.pop_front();
        a.pop_back();
        allpos.erase(al);

        cost += allpos.order_of_key(al);
    }

    cout << cost << '\n';
}

int main() {
#ifndef DEBUG
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
#endif
    int z = 1;
    // cin >> z; // REMOVE TC!
    while (z--)
        solve();
    return 0;
}