#include <bits/stdc++.h>
using namespace std;
const int N = 1e3 + 10;
int n;
string s,pre[N];
int lst[26],ans[26];
bool vis[26],qwq[26];

string work(int pos) {
	for (int i = 0;i < 26;++i) lst[i] = 0,ans[i] = 0,qwq[i] = 0;
	for (int i = pos;i >= 0;--i) {
		if (!lst[s[i] - 'a']) {
			lst[s[i] - 'a'] = i;
			qwq[s[i] - 'a'] = 1;
		}
	}
	//cout << "lst:\n";for (int i = 0;i < 26;++i) cout << lst[i] << " ";cout << "\n";
	for (int i = 0;i < 26;++i) {
		for (int j = 0;j < 26;++j) {
			vis[j] = 0;
		}
		int cnt = 0;
		if (qwq[i]) {
			for (int j = lst[i]+1;j <= pos;++j){
				if (!vis[s[j]-'a']) vis[s[j] - 'a'] = 1,++cnt; 
			}
		}
		ans[i] = cnt;
	}
	
	//cout << "ans:\n";for (int i = 0;i < 26;++i) cout << ans[i] << " ";cout << "\n";
	string nowans = "";
	for (int i = 0;i <= pos;++i) {
		nowans += ('a'+ans[s[i]-'a']);
	}
	return nowans;
}

int main() {
	ios::sync_with_stdio(false);
	
	cin >> n;
	cin >> s;
	for (int i = 0;i < n;++i) {
		pre[i] = work(i);
	}
	sort(pre,pre+n);
	cout << pre[n-1];
}