#include <bits/stdc++.h>
using namespace std;

int main(){
  int n, k;
  string s, ans, tmp;
  cin >> n >> k >> s;
  n = s.size();
  s = "?" + s;
  vector<vector<int>> a(26, vector<int> (n + 1));
  for (int i = 1; i <= n; i++){
    a[s[i] - 97][i]++;
    for (int j = 0; j < 26; j++){
      a[j][i] += a[j][i - 1];
    }
  }
  for (int i = 1; i + k - 1 <= n; ){
    int ok = 1;
    for (int j = 0; j < 26; j++){
      if (a[j][i + k - 1] - a[j][i - 1] > 0 && s[i] - 97 != j){
        for (int l = i; l < i + k; l++){
          tmp += s[l];
        }
        sort(tmp.begin(), tmp.end());
        ans += tmp;
        ok = 0;
        tmp.clear();
        break;
      } else if (a[j][i + k - 1] - a[j][i - 1] > 0 && s[i] - 97 == j){
        break;
      }
    } //cout<<ans<<ok<<endl;
    if (ok){ //out<<s[i]<<endl;
      ans += s[i];
      if (i + k - 1 == n){
        for (int l = i + 1; l <= n; l++){
          tmp += s[l];
        }
        sort(tmp.begin(), tmp.end());
        ans += tmp; //cout<<ans<<endl;
        break;
      } //cout<<ans<<endl;
      i++;
    } else {
      if (n - (i + k - 1) < k){
        for (int l = i + k; l <= n; l++){
          ans += s[l];
        }
        break;
      }
      i = i + k; //cout << i<<endl;
    }
  }
for(int i=0;i<n;i++){
for(int j=0;j<n;j++({
}}
  cout << ans;
}