#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e9 + 7;
const int N = 2e2 + 10;
int a[N];
int main() {
  string str;
  cin >> str;
  memset((a), (0), sizeof(a));
  for (auto &c : str) a[c]++;
  int ptr = 'z';
  for (int i = 'a'; i <= 'z'; i++) {
    if (a[i] % 2 == 0) continue;
    while (a[ptr] % 2 == 0) ptr--;
    a[i]++;
    a[ptr]--;
  }
  int key = 0;
  for (int i = 'a'; i <= 'z'; i++) {
    if (a[i] & 1) key = i;
    a[i] >>= 1;
    int o = a[i];
    while (o--) putchar(i);
  }
  if (key) putchar(key);
  for (int i = 'z'; i >= 'a'; i--) {
    while (a[i]--) putchar(i);
  }
  return 0;
}
