#include <bits/stdc++.h>
using namespace std;
static bool odd(int i) { return i % 2; }
int main() {
  int count[26] = {};
  string S;
  cin >> S;
  for (char ch : S) ++count[ch - 'a'];
  const int L = S.size();
  const int len_odd = odd(L);
  int num_odd = 0;
  for (int i = int(0); i < int(26); ++i) num_odd += odd(count[i]);
  int begin = 0, end = 26;
  while (num_odd > len_odd) {
    while (!odd(count[begin])) ++begin;
    do --end;
    while (!odd(count[end]));
    assert(begin < end);
    ++count[begin];
    --count[end];
    num_odd -= 2;
  }
  assert(num_odd == len_odd);
  int j = 0;
  for (int i = 0; i < L / 2; ++i) {
    while (count[j] < 2) ++j;
    assert(j < 26);
    S[i] = S[L - 1 - i] = char('a' + j);
    count[j] -= 2;
  }
  if (len_odd) {
    for (int j = int(0); j < int(26); ++j)
      if (count[j] > 0) {
        assert(count[j] == 1);
        S[L / 2] = char('a' + j);
        break;
      }
  }
  cout << S << endl;
}
