#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-6;
const int INF = 2e9;
const long long LNF = 9e18;
const int mod = 1e9 + 7;
const int maxn = 2e5 + 10;
int sum[30], len;
char s[maxn];
void init() {
  scanf("%s", s + 1);
  len = strlen(s + 1);
  for (int i = 1; i <= len; i++) sum[s[i] - 'a']++;
}
void solve() {
  int i = 0, j = 25;
  while (i < j) {
    while (i < j && !(sum[i] & 1)) i++;
    while (i < j && !(sum[j] & 1)) j--;
    sum[i]++;
    sum[j]--;
  }
  for (i = 0; i <= 25; i++)
    for (j = 1; j <= sum[i] / 2; j++) printf("%c", i + 'a');
  for (i = 0; i <= 25; i++)
    if (sum[i] % 2) printf("%c", i + 'a');
  for (i = 25; i >= 0; i--)
    for (j = 1; j <= sum[i] / 2; j++) printf("%c", i + 'a');
  cout << endl;
}
int main() {
  init();
  solve();
}
