#include <bits/stdc++.h>

using namespace std;

#define ll long long

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n;cin >> n;
    string s;cin >> s;
    string t;
    set<string,greater<>> q;
    for (auto x : s){
        t += x;
        string a;
        map<char,char> op;
        char now = 'a';
        for (int i = t.size() - 1;i >= 0;i--){
            if (op.count(t[i])){
                a.push_back(op[t[i]]);
            }else{
                op[t[i]] = now++;
                a.push_back(op[t[i]]);
            }
        }
        reverse(a.begin(),a.end());
        q.insert(a);
    }
    cout << *q.begin() << endl;


    return 0;
}