#include<iostream>
#include<string.h>
#include<algorithm>
#include<math.h>
#include<vector>
#include<map>
#include<queue>
#include<set> 
#include<stack>
#include<bitset>
#include<unordered_map>
#include<iomanip>
#include<cassert>
#include<array>
#define fi first
#define se second
#define endl '\n'
#define pb push_back
#define ep emplace_back 
#define debug(x) cout<<x<<'\n'
#define RES cout<<res<<'\n'
#define ANS cout<<ans<<'\n'
#define YES	cout<<"YES"<<'\n'
#define NO cout<<"NO"<<'\n'
#define Yes	cout<<"Yes"<<'\n'
#define No cout<<"No"<<'\n'
#define me(a,x) memset(a,x,sizeof(a))
#define L(i,j,k) for(int i=(j);i<=(k);++i)
#define R(i,j,k) for(int i=(j);i>=(k);--i)
#define r23 cout<<233<<endl;return;
#define int long long 
using namespace std;
typedef long long ll;
typedef pair<int,int> PII;
typedef pair<PII,int> PIII;
typedef unsigned long long ull;
const int N=1e5+5,P=131,mod=998244353;

char son[100]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t'};
string Sr[1010];
int f[1010];
string s;
set<char> st;
void change(int len){
	st.clear();
	for(int i=len;i>=0;i--){
		if(st.count(s[i])==0)f[s[i]]=st.size();
		st.insert(s[i]);
	}
}
//cac aba
void solve(){
	int n;cin>>n;
	cin>>s;
	L(i,1,n){
		change(i-1);
		L(j,0,i-1){
			Sr[i]+=son[f[s[j]]];
		}
	}
	sort(Sr+1,Sr+1+n);
	cout<<Sr[n]<<endl;
}
signed main(){
	ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
	int T=1;
	//cin>>T;
	while(T--){
		solve();
	}
}