#include <bits/stdc++.h>
using namespace std;
int cnt[300];
int main() {
  string str;
  cin >> str;
  memset(cnt, 0, sizeof(cnt));
  for (int i = 0; i < str.length(); i++) cnt[str[i]]++;
  int res = 0;
  int lb = 'a', ub = 'z';
  while (ub > lb) {
    if (cnt[lb] % 2 == 0) {
      lb++;
      continue;
    }
    if (cnt[ub] % 2 == 0) {
      ub--;
      continue;
    }
    if (cnt[lb] % 2 == 1 && cnt[ub] % 2 == 1) {
      cnt[lb++]++;
      cnt[ub--]--;
    }
  }
  string fuck;
  int idx = -1;
  for (int i = 'a'; i <= 'z'; i++) {
    if (cnt[i] == 0) continue;
    if (cnt[i] % 2 == 1) idx = i;
    for (int j = 0; j < cnt[i] / 2; j++) fuck.push_back(i);
  }
  cout << fuck;
  if (idx != -1) cout << (char)idx;
  string fuck2;
  for (int i = fuck.length() - 1; i >= 0; i--) fuck2.push_back(fuck[i]);
  cout << fuck2 << endl;
}
