#include <bits/stdc++.h>
using namespace std;
set<int> Set;
bool flag;
int n, m, Ans, Count, Pos[300010], Pre;
char str[300010], Deft;
inline int gcd(int x, int y) {
  if (!y) return x;
  return gcd(y, x % y);
}
inline void read(int &x) {
  char ch;
  int N = 1;
  while ((ch = getchar()) && (!((ch >= '0') && (ch <= '9'))) && (!(ch == '-')))
    ;
  if ((ch == '-')) {
    N = -1;
    while ((ch = getchar()) && (!((ch >= '0') && (ch <= '9'))))
      ;
  }
  x = ch - 48;
  while ((ch = getchar()) && (((ch >= '0') && (ch <= '9'))))
    x = x * 10 + ch - 48;
  x *= N;
}
inline void Insert(int x) {
  Pos[x] = x;
  Set.insert(x);
}
inline bool cmp(const int &x, const int &y) { return str[x] < str[y]; }
inline void Work(int x) {
  set<int>::iterator Item, Item_;
  Item = Item_ = Set.lower_bound(Pos[x]);
  if ((*Item) - (*(--Item_)) > m) flag = 0;
}
int main() {
  read(m);
  scanf("%s", str + 1);
  n = strlen(str + 1);
  for (int i = 1; i <= n; i++) Insert(i);
  Set.insert(0);
  Set.insert(n + 1);
  sort(Pos + 1, Pos + n + 1, cmp);
  flag = Ans = 1;
  for (int i = n; i >= 1; i--) {
    Set.erase(Pos[i]);
    Work(i);
    if (!flag) {
      Deft = str[Pos[i]];
      break;
    }
  }
  for (int i = 1; i <= n; i++) {
    if (Ans > m) Ans = i - Pre, Count++;
    if (str[i] < Deft) Ans = 0;
    Pre = (str[i] == Deft) ? (i) : (Pre);
    Ans++;
  }
  Count += (Ans > m);
  for (int i = 1; str[Pos[i]] != Deft; i++) printf("%c", str[Pos[i]]);
  for (int i = 1; i <= Count; i++) printf("%c", Deft);
  printf("\n");
}
