#include <bits/stdc++.h>
template <class T>
void RD(T& ret) {
  ret = 0;
  bool ok = 0, u = 0;
  for (;;) {
    int c = getchar();
    if (c >= '0' && c <= '9')
      ret = (ret << 3) + (ret << 1) + c - '0', ok = 1;
    else if (c == '-')
      u = 1;
    else if (ok) {
      if (u) ret *= -1;
      return;
    }
  }
}
long long pow_mod(long long p, long long n, long long mod) {
  long long ret = 1;
  for (; n; n >>= 1) {
    if (n & 1) ret = ret * p % mod;
    p = p * p % mod;
  }
  return ret;
}
template <class T>
bool chmin(T& a, const T& b) {
  return b < a ? a = b, 1 : 0;
}
template <class T>
bool chmax(T& a, const T& b) {
  return b > a ? a = b, 1 : 0;
}
using namespace std;
int _;
char s[1 << 18];
int c[100];
int main() {
  scanf("%s", s);
  int n = strlen(s), cnt = 0;
  for (int i = 0; i < n; ++i) {
    c[s[i] - 'a']++;
  }
  for (int i = 0; i < 26; ++i)
    if (c[i] & 1) ++cnt;
  char tmp = 0;
  bool flag = 0;
  if (cnt & 1) flag = 1;
  cnt >>= 1;
  string ans;
  for (int i = 0; i < 26; ++i) {
    if (c[i] & 1) {
      if (cnt > 0) {
        --cnt;
        ++c[i];
      } else {
        --c[i];
        if (!tmp) tmp = i + 'a';
      }
    }
    c[i] >>= 1;
    while (c[i]--) ans.push_back(i + 'a');
  }
  string ans2 = ans;
  reverse((ans2).begin(), (ans2).end());
  if (flag) {
    ans.push_back(tmp);
  }
  ans += ans2;
  cout << ans << endl;
  return 0;
}
