#include <bits/stdc++.h>  
using namespace std;  

#define int long long  

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

	int n;
	cin >> n;

	string s;
	cin >> s;

	

	auto work = [&](string &x) {
		int cnt[26] {};
		int mp[26] {};
		for (int i = (int)x.size() - 1; i >= 0; i--) {
			if (!cnt[x[i] - 'a']) {
				for (int j = 0; j < 26; j++) {
					if (cnt[j]) {
						mp[x[i] - 'a']++;
					}
				}
			}
			cnt[x[i] - 'a']++;
		}
		for (auto &c : x) {
			c = 'a' + mp[c - 'a'];
		}
	};

	vector<string> a(n);
	for (int i = 0; i < n; i++) {
		a[i] = string(s.begin(), s.begin() + i + 1);
		work(a[i]);
	}

	sort(a.begin(), a.end());
	cout << a[n - 1] << "\n";

    return 0;
}