#include <cstring>
#include <iostream>
#include <algorithm>
#include <queue>
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
#define int long long
typedef long long ll; 

const int N = 1e5+10;
const int mod = 998244353;


void solve()
{
    int n;
    cin>>n;
    string s;
    cin>>s;
    
    
    vector<string>ans;
    for(int i = 1;i<=n;i++)
    {
        string tmp = s.substr(0,i);
        map<char,char>mmp;
        map<char,int>mp;
        int m = tmp.size();
        int cnt = 0;
        for(int j = m-1;j>=0;j--)
        {
            if(mp[tmp[j]]==0)
            {
                mp[tmp[j]] = 1;
                mmp[tmp[j]] = 'a'+cnt;
                cnt++;
            }
        }
        string op = "";
        for(int j = 0;j<m;j++)
        {
            op+=mmp[tmp[j]];
        }
        ans.push_back(op);
    }

    sort(ans.begin(),ans.end());
    string sss = ans[ans.size()-1];

    cout<<sss<<endl;



    return;
}
signed main(){
    int _ = 1;
    ios::sync_with_stdio(0);cin.tie(0),cout.tie(0);
    //cin>>_;
    while(_--){
        solve();
    }
    return 0;
}