#include <cstring>
#include <iostream>
#include <algorithm>
#include <queue>
#include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;
#define int long long
typedef long long ll; 

const int N = 1e5+10;
const int mod = 998244353;


void solve()
{
    int n;
    cin>>n;
    string s;
    cin>>s;
    
    map<char,int>mp;
    map<char,char>mmp;
    int en = n;
    for(int i = 0;i<n;i++)
    {
        if(mp[s[i]]==0)
        {
            mp[s[i]]=1;
        }
        else if(mp[s[i]]==1)
        {
            if(s[i]!=s[i-1])
            {
                en = i;
                break;
            }
        }
    }
    int cnt = 0;
    for(int i = en-1;i>=0;i--)
    {
        if(i==en-1)
        {
            mmp[s[i]] = 'a'+cnt;
            cnt++;
            continue;
        }
        if(s[i]!=s[i+1])
        {
            mmp[s[i]] = 'a'+cnt;
            cnt++;
        }
    }
    bool ok = 0;
    string tmp = s.substr(0,en);
    int op = en;
    for(int i = en;i<n;i++)
    {
        if(ok&&s[i]!=tmp[0])break;
        if(s[i]==tmp[0])
        {
            int j = i;
            int cnt = 0;
            while(j<n&&cnt<en&&s[j]==tmp[cnt])
            {
                j++;
                cnt++;
            }
            
            if(cnt>=en)
            {
                ok = 1;
                op+=en;
                i = j-1;
            }
            else{
                break;
            }
        }
    }


    for(int i = 0;i<op;i++)
    {
        cout<<mmp[s[i]];
    }




    return;
}
signed main(){
    int _ = 1;
    ios::sync_with_stdio(0);cin.tie(0),cout.tie(0);
    //cin>>_;
    while(_--){
        solve();
    }
    return 0;
}