#include<bits/stdc++.h>
using namespace std;
#define int long long
#define ull long long
#define PII  pair<int ,int>
const int INF = -1145141919810;
const int mod = 1e9 + 7;
const int N = 1e6 + 7;
int fp(int a, int x, int mod) {
	int ans = 1;
	while (x) {
		if (x & 1)ans *= a;
		ans %= mod;
		a *= a;
		a %= mod;
		x >>= 1;
	}
	return ans;
}


map<char, map<char,int>>p;
//map<char, int>p;
void solve() {
	int n;
	cin >> n;
	string s;
	cin >> s;
	string ans;
	for (int i = 0; i < s.length(); i++) {
		p[s[i]].clear();
		for (int j = 0; j <= 26; j++) {
			char ch = 'a'+j;
			if(p.count(ch)!=0&&ch!=s[i])p[ch][s[i]]++;
		}
		string res;
		for (int j = 0; j <= i; j++) {
			res += 'a' + p[s[j]].size();
		}
		//cout << res << endl;
		ans = max(ans, res);
	}
	cout << ans << endl;
}
//&&(((sum[n]+k)%mid==0)||(sum[n]/mid!=(sum[n]+k)/mid)||(mid-(sum[n]%mid)>=k))
signed main() {
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	int t = 1;
	//cin >> t;
	//er(9e7);
	//cout <<  7*4% 10;
	while (t--)solve();
	//pr[5216953]
}
//int n;
//cin >> n;
//for (int i = 1; i <= n; i++) {
//	cin >> a[i];
//}
//int maxx = 0, minn = INF;
//int sum = 0;
//for (int i = n; i <= 1; i++) {
//	sum += minn;
//	if (top == 0) {
//		minn = a[i];
//		st[top++] = a[i];
//	}
//	else {
//		if (a[i] > top && a[i] > minn)
//	}
//}
//cout << maxx - minn << endl;
//7 2 1 4 5 1 3 3
//4 1000 1000 1000 1000
//0