#include <iostream>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <vector>
#include <set>
#include <queue>
#include <map>
#define endl '\n'
#define int long long
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define rep2(i,a,b) for(int i=(a);i>=(b);i--)
using namespace std;
template<typename... Args>
void bubu(Args... args) { cout << ":: "; ((cout << args << " "), ...); cout << endl; }
void bubu(vector<char> tem) { for (auto x : tem) cout << x << ' '; cout << endl; }
void kuaidu() { ios::sync_with_stdio(false), cin.tie(0), cout.tie(0); }
inline int max(int a, int b) { if (a < b) return b; return a; }
inline int min(int a, int b) { if (a < b) return a; return b; }
using PII = pair<int, int>;
using i128 = __int128;

//=======================================================================
const int N = 1e5 + 10;
const int M = 1e6 + 10;
const int mod = 1e9 + 7;
const int INF = 1e16;
int n, m, T;

//=======================================================================

signed main() {
	kuaidu();
	cin >> n;
	string s; cin >> s;
	priority_queue<string> F;
	rep(i, 0, n - 1) {
		map<char, int> mp, mp2, mp3;
		vector<char> A;

		string s1; rep(j, 0, i) s1.push_back(s[j]);

		int cnt = 0;
		rep2(j, i, 0) {
			if (mp[s[j]]) continue;
			cnt++;
			mp[s[j]] = j;
			mp2[s[j]] = cnt - 1;
			A.push_back(s[j]);
		}
		sort(A.begin(), A.end(), [&](char a, char b) {
			if (mp2[a] == mp2[b]) return mp[a] > mp[b];
			return mp2[a] < mp2[b];
			});
		// cout << i << " ";
		// bubu(A);
		int l = 0;
		for (auto x : A) mp3[x] = ++l;
		for (auto& x : s1) { x = char(mp3[x] + 'a' - 1); }
		F.push(s1);
	}
	cout << F.top() << endl;
	return 0;
}