#include <bits/stdc++.h>
using namespace std;
int tab[26] = {false}, np = 0;
int main() {
  string s;
  char a, b;
  cin >> s;
  for (int i = 0; i < s.size(); i++) tab[s[i] - 97]++;
  for (int i = 0; i <= 26; i++)
    if (tab[i] % 2 == 1) np++;
  while (np > 1) {
    int i = 0;
    while (tab[i] % 2 == 0) i++;
    int j = 26;
    while (tab[j] % 2 == 0) j--;
    tab[i]++;
    tab[j]--;
    np = np - 2;
  }
  if (s.size() % 2 == 1) {
    int i = 0;
    while (tab[i] % 2 == 0) i++;
    b = i + 97;
  }
  for (int i = 0; i < 26; i++) {
    int k = tab[i] / 2;
    a = i + 97;
    while (k-- > 0 && tab[i] > 0) cout << a;
  }
  if (s.size() % 2 == 1) cout << b;
  for (int i = 25; i >= 0; i--) {
    int k = tab[i] / 2;
    a = i + 97;
    while (k-- > 0 && tab[i] > 0) cout << a;
  }
}
