#include<bits/stdc++.h>
using namespace std;
const int M=1005;
int n;
char S[M],TS[M];
string ans,tmp;
bool flag[M];
int mk[M];
int cnt;
int main() {
    scanf("%d",&n);
    scanf("%s",S+1);
    ans='a';
    for(int i=2; i<=n; ++i) {
        cnt=0;tmp="";
        memset(flag,0,sizeof flag);
        memset(mk,0,sizeof mk);
        for(int j=i; j; --j) {
            if(!flag[S[j]])flag[S[j]]=1,cnt++;
            if(!mk[S[j]])mk[S[j]]=cnt+'a'-1;
            TS[j]=mk[S[j]];
        }
        for(int j=1; j<=i; ++j)tmp=tmp+TS[j];
        if(tmp>ans)ans=tmp;
    }
    cout<<ans;
    return 0;
}