#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n'
#define inf 0x3f3f3f3f3f3f3f3f

const int maxn = 1e5 + 10;
const int mod = 1e9 + 7;

void solve()
{
    int n;
    cin >> n;
    string s;
    cin >> s;
    // int n = s.size();
    s = ' ' + s;
    int cnt = 1;
    int pos = n;
    map<int, int> ma;
    int maxx = 0;
    string ss = "";
    for (int i = n; i >= 1; i--)
    {
        string tt = "";
        for (int j = i; j >= 1; j--)
        {
            if (!ma[s[j] - 'a'])
                ma[s[j] - 'a'] = cnt++;
        }
        for (int j = 1; j <= i; j++)
        {
            tt += char(ma[s[j] - 'a'] + 'a' - 1);
        }
        if (tt >= ss)
            ss = tt;
        for (int j = 0; j <= 26; j++)
        {
            ma[j] = 0;
        }
        cnt = 1;
    }

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