#include<bits/stdc++.h>

using namespace std;

#define pll pair<ll, ll>
#define ll long long
const int mod = 1e9 + 7;
const int N = 3e5 + 50;

void solve(){
    int n;
    cin>>n;
    string s;
    cin>>s;
    s=" "+s;
    string As;
    for(int i=1;i<=n;++i){
       map<char,char>mp;
       set<char>S;
       string as;
       for(int j=i;j>=0;--j){
          if(S.find(s[j])!=S.end()) continue;
          mp[s[j]]=S.size();
          S.insert(s[j]);
       }
       for(int j=1;j<=i;++j){
          as.push_back(char('a'+mp[s[j]]));
       }
       if(as>As) As=as;
    }
    cout<<As<<'\n';
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll T = 1;
    //cin >> T;
    while (T --)
    {
        solve();
    }
    
    return 0;
}