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#include <bits/stdc++.h>

#define INF 2147483647
#define LINF 9223372036854775807
#define NINF -2147483648
#define NLINF -9223372036854775808
#define M 1000000007
#define M1 998244353
#define A 26
#define K 31
#define P 2137

using namespace std;
using db=double;
using dbl=long double;
using ll=long long;
using pi=pair<int,int>;
using pl=pair<ll,ll>;
using vi=vector<int>;
using vl=vector<ll>;
using gr=vector<vector<int> >;
using grl=vector<vector<ll> >;


#define fp(x, a, b) for (int (x) = (a); (x) < (b); (x)++)
#define f(x, n) for (int (x) = 0; (x) < (n); (x)++)
#define fnp(x, a, b) for (int (x) = (b) - 1; (x) >= (a); (x)--)
#define fn(x, n) for (int (x) = (n - 1); (x) >= 0; (x)--)
#define sgn(x) (x) > 0 ? 1 : (x) == 0 ? 0 : -1
#define gcd(a, b) __gcd((a), (b))
#define lcm(a, b) (a) * (b) / gcd((a), (b))
#define x first
#define y second
#define mp make_pair
#define pb push_back
#define s(x) x.size()
#define all(x) x.begin(), x.end()
#define ans(x) cout<<(x)<<"\n"
#define yes cout<<"YES\n";
#define no cout<<"NO\n";
#define fl cout.flush()
#define debarr(x, n) f(i, (n)){cout<<(x)[i]<<" ";}cout<<"\n"
#define debgr(x, n) f(i, (n)){f(j, s((x)[i])){cout<<(x)[i][j]<<" ";}cout<<"\n";}


mt19937 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());

void input();
void compute();

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int T = 1;
    //cin >> T;
    while(T--)
    {
        input();
        compute();
    }
    return 0;
}

#define N 5001

string s;
int n;
ll w;
int cntA;
int cntB;
int d;
vi aPos;

void input()
{
    cin >> s;
    n = s(s);
    w = 0;
    cntA = 0;
    cntB = 0;
    d = 0;
}
void compute()
{
    f(i, n) {
        if(s[i] == 'a') {
            cntA++;
            if(i <= n / 2) {
                d++;
            }
        } else {
            cntB++;
        }
    }
    if((n % 2 == 0) && (cntA % 2 == 1 || cntB % 2 == 1)) {
        ans(-1);
        return;
    }
    if(n % 2 == 1 && (cntA % 2 == 1 && cntB % 2 == 1)) {
        ans(-1);
        return;
    }
    if(d > s(aPos) / 2) {
        reverse(all(s));
    }
    f(i, s(s)) {
        if(s[i] == 'a') {
            aPos.pb(i);
        }
    }
    f(i, s(aPos)) {
        if(i >= s(aPos) / 2) {
            break;
        }
        int k = aPos[i] - (n - aPos[s(aPos) - i - 1] - 1);
        if(k < 0) {
            k *= -1;
        }
        w += k;
    }
    ans(w);
}