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

void solve() {
	int n;
	cin >> n;
	string s;
	cin >> s;
	s = " " + s;
	string ans = "";
	for (int i = 1; i <= n; i++) {
		string t = " " + s.substr(1, i);
		//cout << t << '\n';
		set<char>st;
		vector<int>c(26);
		for (int j = i; j; j--) {
			if (st.find(t[j]) == st.end()) {
				c[t[j] - 'a'] = st.size();
				st.insert(t[j]);
			}
		}
		//cout << '\n';
		for (int j = 1 ; j <= i ; j++) {
			t[j] = c[t[j] - 'a'] + 'a';
		}
		//cout << t << '\n';
		ans = max(ans, t.substr(1));
	}
	cout << ans << '\n';
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int t = 1;
	//cin>>t;
	while (t--)
		solve();
	return 0;
}