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// while (clock()<=69*CLOCKS_PER_SEC)
// #pragma comment(linker, "/stack:200000000")
// #pragma GCC optimize("O3")
// #pragma GCC target ("avx2")
// #pragma GCC optimize("Ofast")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// #pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
using namespace std;

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

#define sim template <class c
#define ris return *this
#define dor > debug &operator<<
#define eni(x)                                                                    \
    sim > typename enable_if<sizeof dud<c>(0) x 1, debug &>::type operator<<(c i) \
    {
sim > struct rge {
    c b, e;
};
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c *x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef XOX
    ~debug()
    {
        cerr << endl;
    }
    eni(!=) cerr << boolalpha << i;
    ris;
} eni(==) ris << range(begin(i), end(i));
}
sim, class b dor(pair<b, c> d)
{
    ris << "" << d.first << " --> " << d.second << "";
}
sim dor(rge<c> d)
{
    *this << "[";
    for (auto it = d.b; it != d.e; ++it)
        *this << ", " + 2 * (it == d.b) << *it;
    ris << "]";
}
#else
    sim dor(const c &)
    {
        ris;
    }
#endif
}
;
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "

#ifdef XOX
#warning Times may differ!!!
#endif

#define endl '\n'
#define pb emplace_back
#define vt vector
#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

const int nax = 1000 * 1007, mod = 1e9 + 7;

#define int long long

int32_t main()
{
    cin.tie(nullptr)->sync_with_stdio(0);
    string s;
    cin >> s;
    int n = sz(s);
    vector<set<int>> gdz(2);
    auto valid = [&]() {
        vector<int> ile(2);
        for (int i = 0; i < n; i++) {
            s[i] -= 'a';
            ile[s[i]]++;
            gdz[s[i]].emplace(i);
        }
        int s = (ile[0] % 2) + (ile[1] % 2);
        return s <= 1;
    };
    auto slop = [&](int i, int j) {
        gdz[s[i]].erase(i);
        gdz[s[i]].emplace(j);
        gdz[s[j]].erase(j);
        gdz[s[j]].emplace(i);
        swap(s[i], s[j]);
    };
    if (!valid()) {
        cout << -1 << endl;
        exit(0);
    }
    int ans = 0;
    for (int i = 0; i < n - i - 1; i++) {
        int j = n - i - 1;
        if (s[i] != s[j]) {
            int wez = *prev(gdz[s[i]].lower_bound(j));
            if (wez == i) {
                ans++;
                slop(i, i + 1);
            }
            else {
                ans += j - wez;
                slop(wez, j);
            }
        }
    }
    for (int i = 0; i < (n - i - 1); i++) {
        assert(s[i] == s[n - i - 1]);
    }
    cout << ans << endl;
}