#include <bits/stdc++.h>
using namespace std;
template <class T>
T bigmod(T a, T b) {
  T ans = 1;
  T base = a % 1000000007LL;
  while (b) {
    if (b & 1) ans = (ans * base) % 1000000007LL;
    base = (base * base) % 1000000007LL;
    b /= 2;
  }
  return ans;
}
template <class T>
T Bitcnt(T a) {
  int sum = 0;
  while (a) {
    if (a & 1) sum++;
    a /= 2;
  }
  return sum;
}
template <class T>
T Max3(T a, T b, T c) {
  return max(a, max(b, c));
}
template <class T>
T Lcm(T a, T b) {
  T tmp = __gcd(a, b);
  return (a / tmp) * b;
}
template <class T>
T Pow(T a, T b) {
  T ans = 1;
  T base = a;
  while (b) {
    if (b & 1) ans = (ans * base);
    base = (base * base);
    b /= 2;
  }
  return ans;
}
int a_x[] = {1, -1, 0, 0};
int a_y[] = {0, 0, 1, -1};
long long X, Y;
void extend_euclid(long long a, long long b) {
  if (b == 0) {
    X = a;
    Y = 0;
    return;
  }
  extend_euclid(b, a % b);
  long long x, y;
  x = Y;
  y = X - (a / b) * Y;
  X = x;
  Y = y;
}
long long inverse_modulo(long long a, long long b) {
  extend_euclid(a, b);
  return (X + 1000000007LL) % 1000000007LL;
}
int cnt[4000000];
char out[4000000];
int main() {
  string inp;
  cin >> inp;
  int len = inp.size();
  for (int i = 0; i < len; i++) cnt[inp[i] - 'a']++;
  for (int i = 25; i >= 0; i--) {
    if (cnt[i] & 1) {
      for (int j = 0; j < i; j++) {
        if (cnt[j] & 1) {
          cnt[j]++;
          cnt[i]--;
          break;
        }
      }
    }
  }
  int l = 0;
  int r = len - 1;
  for (int i = 0; i < 26; i++) {
    if (cnt[i] > 0) {
      while (cnt[i] > 1) {
        out[l++] = i + 'a';
        out[r--] = i + 'a';
        cnt[i] -= 2;
      }
    }
  }
  if (len & 1) {
    for (int i = 0; i < 26; i++) {
      if (cnt[i] & 1) {
        out[len / 2] = i + 'a';
        break;
      }
    }
  }
  out[len] = '\0';
  cout << out << endl;
  return 0;
}
