#include <bits/stdc++.h>
using namespace std;
int inf_int = 2e9;
long long inf_ll = 2e18;
const double pi = 3.1415926535898;
long long binpow(long long a, long long n) {
  if (n == 0) {
    return 1;
  }
  long long res = 1;
  while (n) {
    if (n % 2 == 1) {
      res *= a;
    }
    a *= a;
    n /= 2;
  }
  return res;
}
int gcd(int a, int b) {
  while (b) {
    a %= b;
    swap(a, b);
  }
  return a;
}
const double EPS = 1e-7;
int siz(long long x) {
  if (x < 1e1) {
    return 1;
  } else if (x < 1e2) {
    return 2;
  } else if (x < 1e3) {
    return 3;
  } else if (x < 1e4) {
    return 4;
  } else if (x < 1e5) {
    return 5;
  } else if (x < 1e6) {
    return 6;
  } else if (x < 1e7) {
    return 7;
  } else if (x < 1e8) {
    return 8;
  } else if (x < 1e9) {
    return 9;
  } else if (x < 1e10) {
    return 10;
  }
}
int m;
string s;
map<char, vector<int> > mmap;
vector<int> indices;
vector<bool> triggered;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> m >> s;
  string ans;
  for (int i = 0; i < s.size(); i++) {
    mmap[s[i]].push_back(i);
  }
  triggered.assign(s.size(), 0);
  int mx = -1;
  for (int i = 0; i < s.size() - m + 1;) {
    int mn = 125;
    for (int j = i; j < i + m; j++) {
      mn = min(mn, (int)s[j]);
    }
    mx = max(mn, mx);
    for (int j = i + m - 1; j >= i; j--) {
      if ((int)s[j] == mn) {
        i = j + 1;
        break;
      }
    }
  }
  for (int i = (int)'a'; i < mx; i++) {
    for (int j = 0; j < mmap[(char)i].size(); j++) {
      triggered[mmap[(char)i][j]] = 1;
      ans.push_back((char)i);
    }
  }
  for (int i = 0; i < s.size() - m + 1;) {
    bool c = 0;
    for (int j = i; j < i + m; j++) {
      if (triggered[j] == 1) {
        c = 1;
        i = j + 1;
      }
    }
    if (!c) {
      for (int j = i + m - 1; j >= i; j--) {
        if ((int)s[j] == mx) {
          triggered[j] = 1;
          ans += mx;
          i = j + 1;
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
