#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int a[27] = {0};
  int n = s.length(), r = 0, r1;
  for (auto i = 0; i < n; i++) a[s[i] - 'a']++;
  for (auto i = 0; i < 26; i++) {
    if (a[i] % 2 != 0) r++;
  }
  if (n % 2 != 0 && r > 1) r--;
  r1 = r / 2;
  for (auto i = 25; i >= 0; i--) {
    if (r1 > 0 && a[i] % 2 != 0) {
      a[i]--;
      r1--;
    }
  }
  r1 = r / 2;
  for (auto i = 0; i < 26; i++) {
    if (r1 > 0 && a[i] % 2 != 0) {
      a[i]++;
      r1--;
    }
  }
  int j = 0, i = 0, loc = 26;
  char c[1];
  for (auto j = 0; j < 26; j++) {
    while (i < a[j] / 2) {
      cout << char('a' + j);
      i++;
    }
    i = 0;
    if (a[j] % 2 != 0) c[0] = char('a' + j);
  }
  if (n % 2 != 0) cout << c[0];
  j = 25, i = 0;
  while (j >= 0) {
    while (i < a[j] / 2) {
      cout << char('a' + j);
      i++;
    }
    i = 0;
    j--;
  }
}
