#include <bits/stdc++.h>
using namespace std;
void solve()
{
    int n;
    cin >> n;
    string s;
    cin >> s;
    map<char, set<char>> mp;
    vector<string>ans;
    for(int i=0;i<n;i++)
    {
        for(auto &[key,val]:mp)
        {
            val.insert(s[i]);
        }
        mp[s[i]].clear();
        string mimi;
        for(int j=0;j<=i;j++)
        {
            mimi.push_back(char('a'+mp[s[j]].size()));
        }
        ans.push_back(mimi);
    }
    sort(ans.rbegin(),ans.rend());
    cout<<ans[0]<<endl;
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int T = 1;
    // cin >> T;
    while (T--)
    {
        solve();
    }
    return 0;
}