#include<bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define int long long
const int N=2007;
int n,m;
int f[N],a[N];
string s,cc;
string ss[N];
bool scm(string s1,string s2){
	int i=0,j=0;
	int len1=s1.length();
	int len2=s2.length();
	//cout<<len1<<"!"<<len2<<endl;
	for(;i<len1&&j<len2;i++,j++){
		if(s1[i]-'a'>s2[j]-'a'){
			return true;
		}else if(s1[i]-'a'<s2[j]-'a'){
			return false;
		}
	}
	if(len1>len2) return true;
	return false;
}
void solve(){
	cin>>n;
	cin>>s;
	for(int i=1;i<=n;i++){
		cc+=s[i-1];
		ss[i]=cc;
		//cout<<ss[i]<<"!"<<endl;
	}
	for(int i=1;i<=n;i++){
		for(int j=0;j<=1050;j++){
			a[j]=0x3f3f3f3f;
			f[j]=0;
		}
		//cout<<i<<"!!"<<endl;
		for(int j=i;j>=1;j--){
			//cout<<j<<"!!!"<<f[j]<<endl;
			f[j]=f[j+1];
			if(a[ss[i][j-1]-'a'+1]==0x3f3f3f3f){
				a[ss[i][j-1]-'a'+1]=j;
				f[j]++;
			}
		}
		for(int j=1;j<=i;j++){
			ss[i][j-1]=('a'+f[a[ss[i][j-1]-'a'+1]]-1);
		}
		//cout<<ss[i]<<"!"<<endl;
	}
	string sss=ss[1];
	for(int i=2;i<=n;i++){
		//cout<<sss<<"!"<<ss[i]<<endl;
		if(scm(sss,ss[i])==0){
			sss=ss[i];
		}
	}
	cout<<sss<<endl;
}
signed main(){
	fast;
	int t=1;
	//cin>>t;
	while(t--){
		solve();
	}
}