#include<bits/stdc++.h>


using namespace std;

int main()
{
    int n;
    cin >> n;
    string s;
    cin >> s;
    s = " " + s;
    string s1 = "";
    vector<string> v;
    for(int i = 1; i <= n; i++)
    {
        s1 = s1 +  s[i];
        map<int,int> mp;
        int cnt = 0;
        string ans = "";
        for(int i = s1.size() - 1; i >= 0; i--)
        {
            if(!mp[s1[i]])
            {
                ans += 'a' + cnt;
                mp[s1[i]] = 'a' + cnt;
                cnt++;
            }
            else
            {
                ans += char(mp[s1[i]]);
            }
        }
        reverse(ans.begin(), ans.end());
        v.push_back(ans);
    }
    sort(v.begin(), v.end());
    cout << *v.rbegin() << endl;
    return 0;
}