#include<bits/stdc++.h>

using i64 = long long;

void DAOQI() {
    int n;
    std::string s;
    std::cin >> n >> s;
    std::vector<int> pos(26, -1), doc;
    std::vector<std::string> ans;
    for (int i = 0; i < n; i++) {
        std::string tmp = s.substr(0, i + 1);
        int lst = pos[s[i] - 'a'];
        pos[s[i] - 'a'] = i;
        if (std::binary_search(doc.begin(), doc.end(), lst)) {
            for (int j = 0; j < doc.size(); j++) {
                if (doc[j] == lst) {
                    doc[j] = i;
                    break;
                }
            }
        } else {
            doc.push_back(i);
        }
        std::sort(doc.begin(), doc.end());
        for (int j = 0; j < tmp.size(); j++) {
            int p = std::lower_bound(doc.begin(), doc.end(), pos[tmp[j] - 'a']) - doc.begin();
            p = doc.size() - p-1;
            tmp[j] = p + 'a';
        }
        ans.push_back(tmp);
    }
    std::sort(ans.begin(), ans.end(), std::greater<>());
    std::cout << ans[0] << "\n";
}

signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int T = 1;
    //std::cin >> T;
    while (T--) DAOQI();
    return 0;
}