#include <bits/stdc++.h>

#define all(X) sort((X).begin(), (X).end())
#define int long long
#define f first
#define sec second

#ifdef DEBUG
	#define deb(X) X
	#define log(X) cout << X << "\n"
	#define dump(X) cout << " [" << #X << "] = " << X << " "
#else
	#define deb(X)
	#define log(X)
	#define dump(X)
#endif // DEBUG

using namespace std;
// continue

using ll = long long;
using ull = unsigned long long;
using ld = long double;

void solve();

main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	const auto start = chrono::steady_clock::now();
	solve();
	exit(0);
	const auto finish = chrono::steady_clock::now();
	cout << chrono::duration_cast<chrono::milliseconds > (finish - start).count() << "ms\n";
}

void solve ()
{
	int n;
	cin >> n;
	string s;
	cin >> s;
	map < char, int > mp;
	for (int i = 0; i < s.size(); i++)
	{
		mp[s[i]]++;
	}
	map < char, int > b;
	for (int i = 0; i < s.size(); i++)
	{
		if (b[s[i]])
		{
			s.erase(i, 1);
			i--;
		}
		if (mp[s[i]] == 1){
			continue;
		}
		int cnt = 1, check = 0;
		int ind = i + 1;
		while (cnt != mp[s[i]] && ind < s.size())
		{
			if (s[ind] == s[i])cnt++;
			if (s[ind] < s[i])
			{
				check = 1;
				break;
			}
			if (s[ind] > s[i])
			{
				check = 0;
				break;
			}
			ind++;
		}
		if (!check)
		{
			b[s[i]] = 1;
		}
		if (check)
		{
			mp[s[i]]--;
			s.erase(i, 1);
			i--;
		}
	}
	cout << s;
}