1 #define bug(x) cout<<#x<<" is "<<x<<endl
2 #define IO std::ios::sync_with_stdio(0)
3 #include <bits/stdc++.h>
4 using namespace std;
5 typedef long long ll;
6 #define mk make_pair
7 #define pb push_back
8 const int inf=2147483647;
9 const int N=1e5+10;
10 int k;
11 int a[N][26];
12 int id[200];
13 char s[N];
14 int main(){
15 while(~scanf("%d",&k)){
16 scanf("%s",s+1);
17 int n=strlen(s+1);
18 for(int i=0;i<26;i++){
19 a[n+1][i]=n+1;
20 }
21 for(int i=n;i>=0;i--){
22 for(int j=0;j<26;j++){
23 if(s[i]==j+'a')a[i][j]=i;
24 else a[i][j]=a[i+1][j];
25 }
26 }
27 int i=0,cnt=1;
28 for(;i<=n&&cnt<=k;){
29 for(int j=0;j<26;j++){
30 int x=a[i][j];
31 if(x<=n-k+cnt){
32 cnt++;
33 printf("%c",j+'a');
34 i=x+1;
35 break;
36 }
37 }
38 }
39 printf("\n");
40 }
41 }
