#pragma GCC optimize(2)
#pragma GCC optimize(3, "Ofast", "inline")
#include <bits/stdc++.h>
using namespace std;
#define x first
#define y second
#define int long long
#define endl '\n'
using i128 = __int128;
using i64 = long long;
using f128 = long double;
using u64 = unsigned long long;
using pii = pair<int, int>;
const int INF = 0x3f3f3f3f, mod = 1e9 + 7;
const i64 inf = 2e18;
//-------------------------------------------
void solve()
{
    int n;
    cin >> n;
    string s;
    cin >> s;
    int sz = s.size();
    s = " " + s;
    string ans;
    for (int i = sz; i >= 1; i--)
    {
        vector<int> id(26, -1), st(26);
        int cnt = 0;
        string ss;
        for (int j = i; j >= 1; j--)
        {
            int k = s[j] - 'a';
            if (id[k] == -1)
                id[k] = cnt;
            if (!st[k])
            {
                st[k] = true;
                cnt++;
            }
            ss += 'a' + id[k];
        }
        reverse(ss.begin(), ss.end());
        ans = max(ans, ss);
    }
    cout << ans << endl;
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int T = 1;
   // cin >> T;
    for (int i = 1; i <= T; i++)
        solve();
    return 0;
}