#include<bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define F first
#define S second
signed main(){
	int G[145];	
	int n;
	map < char, int > mp;
	set < char > se;
	set < string > set;
	string s, curr = "";
	cin >> n >> s;
	   
	for(int j = 0; j < n; j ++ ){
		mp.clear();
		se.clear();
		string temp = "";
		for(int i = j; i >= 0; i -- ){
			if( mp[ s[i] ] == 0 ){
				mp[ s[i] ] = se.size()+1;
			}
			temp += 'a' + mp[s[i]]-1;
			se.insert( s[i] );
		}	
		reverse( temp.begin(), temp.end() );
//		cout << j << ":" << temp << "\n";
		set.insert( temp );
	}
	cout << *set.rbegin() << "\n";
}