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//
#ifndef __SIZEOF_INT128__
#define __SIZEOF_INT128__
#endif
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
using namespace std;
using namespace chrono;
using namespace __gnu_pbds;
template <typename T> using oset =  tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define rep(i, p, k) for(int i(p); i < k; ++i)
#define per(i, p, k) for(int i(p); i > k; --i)
#define sz(x) (int)x.size()
#define sc static_cast
typedef long long ll;
typedef long double ld;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef __int128_t lll;
//#define int ll
template <typename T = int> using par = std::pair <T, T>;
#define fir first
#define sec second
#define test int _number_of_tests(in()); while(_number_of_tests--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pb emplace_back
struct Timer {
    string name{""};
    time_point<high_resolution_clock> end, start{high_resolution_clock::now()};
    duration<float, std::milli> dur;
    Timer() = default;
    Timer(string name): name(name) {}
    ~Timer() {
        end = high_resolution_clock::now(); dur= end - start;
        cout << "@" << name << "> " << dur.count() << " ms" << '\n';
    }
};
template <typename T = int> inline T in()
{
    T x;
    std::cin >> x;
    return x;
}
std::string yn(bool b)
{
    if(b) return "YES\n";
    else return "NO\n";
}
template <typename F, typename S> std::ostream& operator<<(std::ostream& out, const std::pair <F, S>& par);
template <typename T> std::ostream& operator<< (std::ostream& out, const std::vector <T>& wek)
{
    for(const auto& i : wek)out << i << ' ';
    return out;
}
template <typename F, typename S> std::ostream& operator<<(std::ostream& out, const std::pair <F, S>& par)
{
    out << '{'<<par.first<<", "<<par.second<<"}";
    return out;
}
#define show(x) cout << #x << " = " << x << '\n';
std::int32_t main()
{
    std::cin.tie(nullptr); std::cout.tie(nullptr); std::ios_base::sync_with_stdio(0);
    string s(in<string>());
    char c;
    if(count(all(s), 'a') % 2 == 0)c = 'a';
    else if(count(all(s), 'b') % 2 == 0)c = 'b';
    else{
        cout << "-1\n";
        return 0;
    }
    vector <int> l, p;
    rep(i, 0, sz(s))if(s[i] == c){
        if(i < sz(s)/2)l.pb((sz(s)-1)/2 - i);
        else p.pb(i - sz(s)/2);
    }
    reverse(all(l));
    if(p.size() < l.size())swap(l, p);
    // cerr << c << ":\n" << l <<"< " << p << '\n';
    ll odp(0);
    for(int i(0); sz(l)+i < sz(p); i += 2)odp += p[i] + p[i+1] + !(sz(s) % 2);
    rep(i, 1, sz(l)+1)odp += abs(l[sz(l)-i] - p[sz(p)-i]);
    cout << odp << '\n';
    return 0;
}