#include <bits/stdc++.h>
using namespace std;
const int maxn = 222222;
char ss[maxn];
int cnt[33], tot, Q[33];
int main() {
  scanf("%s", ss + 1);
  int N = strlen(ss + 1);
  for (int i = 1; i <= N; ++i) ++cnt[ss[i] - 'a'];
  for (int i = 0; i < 26; ++i)
    if (cnt[i] & 1) Q[++tot] = i;
  int uuu = tot >> 1;
  for (int i = 1; i <= uuu; ++i) ++cnt[Q[i]], --cnt[Q[tot - i + 1]];
  if (tot & 1)
    --cnt[uuu = Q[tot - uuu]];
  else
    uuu = -1;
  for (int i = 0; i < 26; ++i)
    for (int j = cnt[i] >> 1; j; --j) putchar('a' + i);
  if (~uuu) putchar('a' + uuu);
  for (int i = 25; ~i; --i)
    for (int j = cnt[i] >> 1; j; --j) putchar('a' + i);
  return 0;
}
