#include <bits/stdc++.h>
using namespace std;
const int INF = 1 << 30;
const int N = 1e5 + 10;
char s[N];
int pre[N][26];
int main() {
  int m, n;
  scanf("%d%s", &m, s);
  n = strlen(s);
  for (int k = 0; k < 26; ++k) {
    int last = -1;
    for (int i = 0; i < n; ++i) {
      if (s[i] - 'a' == k) last = i;
      pre[i][k] = last;
    }
    pre[n][k] = last;
  }
  vector<int> A = {-1, n};
  char c;
  vector<char> ret;
  for (c = 'a'; c <= 'z'; ++c) {
    int cnt = 0;
    vector<int> B = A;
    for (int i = 0; i < n; ++i) {
      if (s[i] == c) {
        B.push_back(i);
        ++cnt;
      }
    }
    sort(B.begin(), B.end());
    bool found = false;
    for (int i = 1; i < B.size(); ++i) {
      if (B[i] - B[i - 1] > m) {
        found = true;
        break;
      }
    }
    if (!found) break;
    for (int i = 0; i < cnt; ++i) ret.push_back(c);
    A = move(B);
  }
  for (int i = 0; i < A.size() - 1; ++i) {
    if (A[i + 1] - A[i] <= m) continue;
    for (int k = A[i]; k + m < A[i + 1];) {
      k = pre[k + m][c - 'a'];
      ret.push_back(c);
    }
  }
  for (auto& it : ret) {
    putchar(it);
  }
  puts("");
  return 0;
}
