/* Problem URL: https://codeforces.com/contest/600/problem/C */

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

using namespace std;
using namespace __gnu_pbds;

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

#define V vector

#define rmin(a, b) a = min(a, b)
#define rmax(a, b) a = max(a, b)

#define rep(i, lim) for (int i = 0; i < (lim); i++)
#define nrep(i, s, lim) for (int i = s; i < (lim); i++)

#define repv(i, v) for (auto &i : (v))
#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
#define sortv(v) sort(v.begin(), v.end())
#define all(v) (v).begin(), (v).end()

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;

using ll = long long;

using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;

template<class v>
auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
    os << vec[0];
    for (size_t i = 1; i < vec.size(); i++) {
        os << ' ' << vec[i];
    }
    os << '\n';
    return os;
}

template<class v>
auto operator>>(istream &is, vector<v> &vec)->istream& {
    for (auto &i : vec) {
        is >> i;
    }
    return is;
}

template<class v>
auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
    for (auto &i : vec) {
        os << i[0];
        for (size_t j = 1; j < i.size(); j++) {
            os << ' ' << i[j];
        }
        os << '\n';
    }
    return os;
}

template<class v>
auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
    for (auto &i : vec) {
        for (auto &j : i) {
            is >> j;
        }
    }
    return is;
}

const int oo = INT32_MAX >> 1;
const ll OO = INT64_MAX >> 1;


void pre()
{

}

#define TEST 0

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

    int n = a.size();

    vi count(26);

    repv(i, a) {
        count[i - 'a']++;
    }

    vi odds;
    rep(i, 26) {
        if (count[i] & 1) {
            odds.push_back(i);
        }
    }

    rep(i, odds.size() >> 1) {
        count[odds[i]]++;
        count[odds[odds.size() - i - 1]]--;
    }

    vi now = count;
    now[0] >>= 1;

    int cur = 0;
    rep(i, n >> 1) {
        while (!now[cur]) {
            cur++;
            now[cur] >>= 1;
        }

        a[i] = cur + 'a';
        now[cur]--;
    }

    if (n & 1) {
        a[n >> 1] = odds[odds.size() >> 1] + 'a';
    }

    count[25] >>= 1;
    cur = 25;
    nrep(i, (n >> 1) + (n & 1), n) {
        while (!count[cur]) {
            cur--;
            count[cur] >>= 1;
        }

        a[i] = cur + 'a';
        count[cur]--;
    }

    cout << a << '\n';
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    pre();

    int t;
    (TEST && cin >> t) || (t = 1);
    while (t--) {
        solve();
    }
}