#include <bits/stdc++.h>
static const int MAXN = 100005;
int n = 0, m, ans_len = 0;
char s[MAXN], ans[MAXN] = {'\0'};
std::pair<char, int> p[MAXN];
std::set<int> marked;
std::multiset<int> distance;
int main() {
  scanf("%d", &m);
  getchar();
  while ((s[n] = getchar()) >= 'a' && s[n] <= 'z') ++n;
  for (int i = 0; i < n; ++i) p[i] = std::make_pair(s[i], i);
  std::sort(p, p + n);
  marked.insert(-1);
  marked.insert(n);
  distance.insert(n + 1);
  for (char c = 'a'; c <= 'z'; ++c) {
    for (int i = 0; i < n; ++i)
      if (p[i].first == c) {
        std::set<int>::iterator prev = marked.lower_bound(p[i].second),
                                succ = prev--;
        int j = i + 1;
        while (j < n && p[j].first == c && p[j].second < *succ) ++j;
        int last_marked = *prev;
        if (*succ <= last_marked + m) continue;
        for (int k = i; k < j; ++k) {
          while (k < j && p[k].second <= last_marked + m) ++k;
          --k;
          if (k < i || p[k].second == last_marked) break;
          prev = marked.lower_bound(p[k].second), succ = prev--;
          ans[ans_len++] = p[k].first;
          marked.insert(p[k].second);
          distance.erase(distance.find(*succ - *prev));
          distance.insert(*succ - p[k].second);
          distance.insert(p[k].second - *prev);
          last_marked = p[k].second;
          if (*succ <= last_marked + m) break;
        }
        i = j - 1;
      }
    if (*(distance.rbegin()) <= m) break;
    for (int i = 0; i < n; ++i)
      if (p[i].first == c) {
        if (marked.find(p[i].second) != marked.end()) continue;
        std::set<int>::iterator prev = marked.lower_bound(p[i].second),
                                succ = prev--;
        ans[ans_len++] = p[i].first;
        marked.insert(p[i].second);
        distance.erase(distance.find(*succ - *prev));
        distance.insert(*succ - p[i].second);
        distance.insert(p[i].second - *prev);
      }
  }
  puts(ans);
  return 0;
}
