#include <bits/stdc++.h>

using namespace std;

using i64 = long long;

string solve(string s) {
    string ans = "";
    vector<int> lst(26, -1), cnt(26, 0);
    for (int i = s.size() - 1; i >= 0; i -= 1) {
        if (lst[s[i] - 'a'] == -1) {
            lst[s[i] - 'a'] = i;
        }
    }
    for (int i = 0; i < 26; i += 1) {
        if (lst[i] == -1) continue;
        int vis[26]{};
        for (int j = lst[i] + 1; j < s.size(); j += 1) {
            vis[s[j] - 'a'] = 1;
        }
        for (int j = 0; j < 26; j += 1) {
            cnt[i] += vis[j];
        }
    }
    for (auto x : s) {
        ans.push_back(cnt[x - 'a'] + 'a');
    }
    return ans;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n;
    cin >> n;

    string s;
    cin >> s;

    string res = "";

    for (int i = 1; i <= n; i += 1) {
        string t = s.substr(0, i);
        res = max(res, solve(t));
    }
    cout << res << "\n";

    return 0;
}