#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double db;
const int N=200010;
const ll mod=998244353;
//const ll mod=1e9+7;
#define rep(i,a,n) for (int i=a;i<=n;i++)
#define per(i,a,n) for (int i=n;i>=a;i--)
#define vi vector <int>
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define sz(a) ((int) (a).size())
#define me(f, x) memset(f, x, sizeof(f))
#define uint unsigned int
#define ull unsigned long long 
#define i128 __int128
#define pb emplace_back
#define pii pair<int,int>
#define debug(x) std::cout<<x<<endl
#define endl '\n'
#define mt19937 rng(chrono::steady_clock::now().time_since_epoch().count())
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
template<typename type>
inline void read(type &x)
{
    x=0;bool flag(0);char ch=getchar();
    while(!isdigit(ch)) flag=ch=='-',ch=getchar();
    while(isdigit(ch)) x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    flag?x=-x:0;
}
template<typename type>
inline void write(type x,bool mode=1)//0为空格，1为换行
{
    x<0?x=-x,putchar('-'):0;static short Stack[50],top(0);
    do Stack[++top]=x%10,x/=10; while(x);
    while(top) putchar(Stack[top--]|48);
    mode?putchar('\n'):putchar(' ');
}
//head~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//begin~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//end~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
string s,ans;
int vis[26];
void solve(){
	int n;
	cin>>n;
	cin>>s;
	n=s.length();
	s=" "+s;
	for(int j=1;j<=n;j++){
		string node=s.substr(1,j);
		memset(vis,0,sizeof vis);
		int cnt=0;
		for(int i=node.size()-1;i>=0;i--){
			if(vis[node[i]-'a']==0){
				vis[node[i]-'a']=cnt+1;
				cnt++;
			}
		}
		for(int i=0;i<node.size();i++){
			node[i]='a'+vis[node[i]-'a']-1;
		}
		if(ans==""){
			ans=node;
		}else{
			if(ans<node){
				ans=node;
			}
		}
	}

	cout<<ans;
}


int main() 
{
	std::ios::sync_with_stdio(false);
	cin.tie(0); 
	cout.tie(0);
	int _=1;
	while(_--){
		solve();
	}
    return 0;
}