#include <bits/stdc++.h>
using namespace std;
int cnt[26];
int main() {
  char str[200001], ans[200001];
  scanf("%s", &str);
  vector<int> v;
  int len = strlen(str), start, last;
  for (int i = 0; i < len; i++) cnt[str[i] - 'a']++;
  for (int i = 0; i < 26; i++)
    if (cnt[i] % 2 == 1) v.push_back(i);
  for (int i = 0; i < v.size() / 2; i++) {
    cnt[v[i]]++;
    cnt[v[(int)v.size() - 1 - i]]--;
  }
  ans[len] = '\0';
  start = 0, last = len - 1;
  for (int i = 0; i < 26; i++) {
    if (cnt[i] % 2 == 1) {
      ans[len / 2] = (char)('a' + i);
      cnt[i]--;
    }
    while (cnt[i] > 0) {
      ans[start] = (char)('a' + i);
      ans[last] = (char)('a' + i);
      start++;
      last--;
      cnt[i] -= 2;
    }
  }
  printf("%s\n", ans);
}
