#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization("unroll-loops")
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
int main() {
  string s;
  cin >> s;
  int n = s.length();
  vector<int> cnt(26, 0);
  for (int i = 0; i < n; i++) {
    cnt[s[i] - 'a']++;
  }
  int st = 0;
  int en = 25;
  for (int i = 0; i < 25; i++) {
    if (cnt[st] % 2 && cnt[en] % 2) {
      cnt[st++]++;
      cnt[en--]--;
    } else if (cnt[st] % 2)
      en--;
    else
      st++;
    ;
  };
  string ans = "";
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < cnt[i] / 2; j++) ans += (char)('a' + i);
  };
  for (int i = 25; i >= 0; i--) {
    if (cnt[i] % 2) ans += (char)('a' + i);
  };
  for (int i = 25; i >= 0; i--) {
    for (int j = 0; j < cnt[i] / 2; j++) ans += (char)('a' + i);
  }
  cout << ans << "\n";
}
