#include <bits/stdc++.h>

using i64 = long long;

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

    int n;
    std::string s;
    std::cin >> n >> s;

    std::string ans = "a";
    for (int i = 0; i < n; i++) {
        std::map<char, int> f;
        for (int j = i; j >= 0; j--) {
            if (!f.count(s[j])) {
                f[s[j]] = f.size();
            }
        }
        std::string res = "";
        for (int j = 0; j <= i; j++) {
            res += char(f[s[j]] + 'a');
        }

        ans = std::max(ans, res);
    }

    std::cout << ans << "\n";
}