#include <bits/stdc++.h>

std::string dfs(std::string s) {
	if(s.size() == 0) return "";
	std::bitset<20> bs;
	int n = s.size(), i, ue, cc;
	for(char c: s) bs[c - 'a'] = 1;
	cc = bs.count();
	bs.reset();
	for(i = n - 1; i >= 0 && bs.count() < cc; --i) bs[s[i] - 'a'] = 1;
	ue = i + 1;
	std::vector<int> occur(20, 0);
	for(i = 0; i <= ue; ++i) occur[s[i] - 'a'] += 1;
	int mo = occur[0]; std::vector<int> maxOccur = {0};
	for(i = 1; i < 20; ++i) {
		if(occur[i] > mo) mo = occur[i], maxOccur.clear(), maxOccur.push_back(i);
		else if(occur[i] == mo) maxOccur.push_back(i);
	}
	// for(int i = 0; i < 20; ++i) std::cerr << occur[i] << char(i == 19 ? 10 : 32);
	std::string pre = "", res = "";
	for(i = 0; i < mo; ++i) pre += char('a' + cc - 1);
	// std::cerr << "pre = " << pre << char(10);
	for(int rem: maxOccur) {
		std::string ss = "";
		int lasOccur = -1;
		for(i = 0; i <= ue; ++i) if(s[i] == rem + 'a') lasOccur = i;
		for(i = lasOccur + 1; i < n; ++i) if(s[i] != rem + 'a') ss += s[i];
		std::string subRes = dfs(ss);
		if(subRes > res) std::swap(res, subRes); 
	}
	return pre + res;
}

int main() {
	int n;
	std::string s;
	std::cin >> n >> s;
	std::cout << dfs(s) << char(10);
	return 0;
}