#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
bool umin(T1 &x, const T2 &y) {
  return (x > y ? x = y, true : false);
}
template <typename T1, typename T2>
bool umax(T1 &x, const T2 &y) {
  return (x < y ? x = y, true : false);
}
const int N = (int)1e6 + 10;
const int INF = (int)1e9 + 17;
const int mod = (int)1e9 + 7;
const long long LLINF = (long long)1e18 + 17;
int n;
string str;
int cnt[400];
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n;
  cin >> str;
  for (auto c : str) {
    cnt[c]++;
  }
  vector<pair<int, int> > p;
  for (int c = 'a'; c <= 'z'; ++c) {
    p.emplace_back(cnt[c], c);
  }
  sort((p).begin(), (p).end());
  for (int i = 0; i < 26; ++i) {
    for (int j = 0; j < p[i].first; ++j) {
      cout << char(p[i].second);
    }
  }
  return 0;
}
