#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <algorithm>
#include <set>
#include <queue>
#include <stack>
#include <map>
#include <unordered_map>
#include <vector>
#define INF 0x3f3f3f3f
#define debug(x) std::cout<<#x<<":"<<x<<endl
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef double db;
const int N=1e6;

signed main() {
    #ifndef ONLINE_JUDGE
        freopen("/Users/holden/Desktop/code/cin.in", "r", stdin);
        freopen("/Users/holden/Desktop/code/cout.out", "w", stdout);
    #endif

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    cin >> n;
    string s;
    cin >> s;
    string now;
    vector<string> sv;
    vector<pii> la(21);
    for (int i = 0; i <= 20; i++) {
        la[i].first = -1;
        la[i].second = i;
    }
    for (int i = 0; i < n; ++i) {
        now += s[i];
        for (int j = 0; j <= 20; ++j) {
            if (la[j].second == s[i] - 'a') {
                la[j].first = i;
                break;
            }
        }
        vector<char> to(21);
        sort(la.begin(), la.end(), [](pii x, pii y) {
            return x > y;
        });
        //cout << la[0].second << '\n';
        for (int j = 0; j <= 20; ++j) {
            to[la[j].second] = (char)(j + 'a');
            //cout << la[j].second << '\n';
            //cout << (char)(j + 'a')<< '\n';
        }
        //cout << to[la[0].second] << '\n';
        string tmp;
        for (int j = 0; j <= i; ++j) {
            tmp += to[s[j] - 'a'];
        }
        sv.push_back(tmp);
    }
    sort(sv.begin(), sv.end(), [](string x, string y) {
        return x > y;
    });
    cout << sv[0];
    return 0;
}