#include <bits/stdc++.h>
using namespace std;
char st[100005];
int n, m, a[100005], f[100005], g[100005];
int main() {
  int i, j, p, q;
  scanf("%d\n", &m);
  scanf("%s", st + 1);
  n = strlen(st + 1);
  for (i = 'a'; i <= 'z'; i++) {
    for (j = 1; j <= n; j++) a[j] = (st[j] == i);
    for (j = 1; j <= n; j++) g[j] = f[j] | a[j];
    p = 0, g[n + 1] = 1;
    for (j = 1; j <= n + 1; j++)
      if (g[j]) {
        if (j - p > m) break;
        p = j;
      }
    if (j <= n + 1) {
      for (j = 1; j <= n; j++) {
        if (a[j]) printf("%c", i);
        f[j] = g[j];
      }
      continue;
    } else {
      p = q = 0;
      for (j = 1; j <= n + 1; j++) {
        if (j - p > m) {
          p = q, q = 0;
          printf("%c", i);
        }
        if (f[j]) {
          p = j, q = 0;
        }
        if (a[j]) q = j;
      }
      return 0;
    }
  }
  return 0;
}
