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/*
    Jakub Żojdzik
    06-12-2022
*/

#include <bits/stdc++.h>
#ifdef LOCAL
#include "debug.hpp"
#else
#define debug(...)
#define dt
#endif
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef double db;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
typedef vector<int> vi;
typedef vector<long long> vll;
typedef vector<pair<int, int>> vpii;
typedef vector<pair<long long, long long>> vpll;

#define fi first
#define se second
#define pb push_back
#define rep(i, x, y) for(ll i = (ll)x; i <= (ll)y; i++)
#define all(x) x.begin(), x.end()
#define sz(x) (ll)(x).size()
#define nl cout << '\n'

vi pos;

int main()
{
    cout.tie(0);
    cin.tie(0)->sync_with_stdio(0);

    string s;
    cin >> s;
    int a = 0, b = 0;
    for(int i = 0; i < sz(s); i++)
    {
        if(s[i] == 'a')
        {
            a++;
            pos.pb(i);
        }
        if(s[i] == 'b')
            b++;
    }
    if(a % 2 == 1 && b % 2 == 1)
    {
        cout << "-1\n";
        return 0;
    }
    int res = 0;
    for(int i = 0; i < sz(pos)/2; i++)
    {
        res += abs(sz(s) - pos[i] - pos[sz(pos)-i-1] - 1);
        // cout << "dodaje " << abs(sz(s) - pos[i] - pos[sz(pos)-i-1] - 1) << '\n';
    }
    if(sz(pos) % 2 == 1)
    {
        res += abs(sz(s)/2 - pos[sz(pos)/2]);
        // cout << "dodatkowo " << abs(sz(s)/2 - pos[sz(pos)/2]) << '\n';
    }
    cout << res << '\n';
}

/*

*/