#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
int n;
char ch[N];
int cnt[26];
string Get() {
  int k = -1, c = 0;
  for (int i = 0; i < 26; ++i) {
    if (cnt[i] & 1) k = i;
    c += cnt[i];
  }
  if (!c) return "";
  string s;
  for (int i = 0; i < 26; ++i) {
    if ((cnt[i] & 1) && i != k) s += char(i + 97);
    cnt[i] /= 2;
  }
  string t = Get();
  s = s + t;
  if (~k) s += char(k + 97);
  reverse(t.begin(), t.end());
  s = s + t;
  return s;
}
int main() {
  scanf("%d", &n);
  scanf("%s", ch + 1);
  sort(ch + 1, ch + n + 1);
  printf("%s\n", ch + 1);
  return 0;
}
