#include <bits/stdc++.h>
using namespace std;
int ABS(int a) { return max(a, -a); }
long long ABS(long long a) { return max(a, -a); }
double ABS(double a) { return max(a, -a); }
void solve() {
  string s;
  cin >> s;
  vector<int> freq(26, 0);
  int n = (int)s.size();
  for (char c : s) {
    freq[c - 'a']++;
  }
  vector<int> odd;
  for (int i = 0; i < 26; i++) {
    if (freq[i] & 1) {
      odd.push_back(i);
    }
  }
  int l = 0, r = (int)odd.size() - 1;
  while (l < r) {
    int i = odd[l];
    int j = odd[r];
    freq[i]++;
    freq[j]--;
    l++;
    r--;
  }
  string t = "", t1 = "", t2 = "";
  char c = '.';
  for (int i = 0; i < 26; i++) {
    if (freq[i] == 0) {
      continue;
    }
    int x = freq[i];
    if (x & 1) {
      c = (i + 'a');
      t2 = c;
      freq[i]--;
      x--;
    }
    x = x / 2;
    for (int j = 0; j < x; j++) {
      t += (i + 'a');
    }
  }
  t1 = t;
  reverse(t1.begin(), t1.end());
  t = t + t2 + t1;
  cout << t;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) solve();
}
