#include<bits/stdc++.h>
using namespace std;
#define V vector
#define pb push_back
#define all0(x) (x).begin(), (x).end()

typedef long long LL;

const int MN = 1e5 + 10;

int main() {
    int n; cin >> n;
    
    string s; cin >> s;
    int ocur;
    V<int> mat(26);

    string ans = "";

    auto enc = [&](string s) -> string {
        for(auto &x : mat) x = -1;
        ocur = 0;

        string ans;

        for(int i = s.size() - 1; i >= 0; i--) {
            if(mat[s[i] - 'a'] != -1) ans.pb(mat[s[i] - 'a'] + 'a');
            else {
                mat[s[i] - 'a'] = ocur;
                ocur++;

                ans.pb(mat[s[i] - 'a'] + 'a');
            }
        }
        reverse(all0(ans));
        return ans;
    };

    for(int i = 1; i <= n; i++) {
        string t = s.substr(0, i);
        ans = max(ans, enc(t));
    }
    

    cout << ans << endl;
}