#include "bits/stdc++.h"

using namespace std;


int main() {
    int n;
    int T = 20;
    int cnt = 1 << 20;
    cin >> n;
    string s;
    cin >> s;
    s = " " + s;

    vector<vector<int>> sums(n + 2, vector<int>(T + 1));
    vector<int> last(T + 1, 0);
    vector<vector<int>> pre(n + 2, vector<int>(T + 1));
    int goals = 0;
    // 20 N
    for (int i = 1; i <= n; i++) {
        goals |= 1 << (s[i] - 'a');
        sums[i] = sums[i - 1];
        sums[i][s[i] - 'a']++;
        pre[i] = pre[i - 1];
        pre[i][s[i] - 'a'] = i;
        last[s[i] - 'a'] = i;
    }
    sums[n + 1] = sums[n];
    pre[n + 1] = pre[n];
    //最小值
    vector<int> f(cnt, n + 1);
    // 2^20
    for (int i = 1; i < cnt; i++) {
        for (int j = 0; j < T; j++) {
            if (i & (1 << j)) {
                int t = min(f[i ^ (1 << j)], last[j]);
                f[i] = min(f[i], t);
            }
        }
    }
    vector<pair<int, vector<int>>> ff(cnt + 1);
    vector<int> vis(cnt);
    vis[0] = 1;
    ff[0] = {0, {}};
    auto gets = [&](int l, int r, int c) {
        if (l > r) return 0;
        return sums[r][c] - sums[l - 1][c];
    };
    auto run = [&](vector<tuple<int, int, int>> &can_touch) {

        auto [las, _, __] = can_touch[0];
        int siz = ff[las].second.size();
        for (int i = 0; i < siz && can_touch.size() > 1; i++) {
            vector<tuple<int, int, int>> tmp;
            int maxs = -1;
            for (auto [x, y, z]: can_touch) {
                maxs = max(maxs, ff[x].second[i]);
            }
            for (auto [x, y, z]: can_touch) {
                if (ff[x].second[i] == maxs) {
                    tmp.emplace_back(x, y, z);
                }
            }
            can_touch = tmp;
        }
        int maxs = -1;
        for (auto [x, y, z]: can_touch) {
            maxs = max(maxs, y);
        }
        vector<tuple<int, int, int>> tmp;
        for (auto [x, y, z]: can_touch) {
            if (y == maxs) {
                tmp.emplace_back(x, y, z);
            }
        }

        maxs = 1e9;
        for (auto [x, y, z]: tmp) {
            maxs = min(maxs, z);
        }

        tuple<int, int, int> ans;
        for (auto [x, y, z]: tmp) {
            if (z == maxs) {
                ans = {x, y, z};
            }
        }
        return ans;
    };
    for (int i = 1; i < cnt; i++) {
        int las = f[goals ^ i];
        if (las == n + 1 && i != goals) continue;
        vector<tuple<int, int, int>> can_touch;
        for (int j = 0; j < T; j++) {
            int t = 1 << j;
            if (i & t && vis[i ^ t]) {
                int l = ff[i ^ t].first + 1;
                int r = pre[las][j];
                int get = gets(l, r, j);
                if (get) can_touch.emplace_back(i ^ t, get, r);
            }
        }
        if (can_touch.empty()) continue;
        else {
            vis[i] = 1;
            auto [lass, add, r] = run(can_touch);
            ff[i] = {r, ff[lass].second};
            ff[i].second.emplace_back(add);
        }
    }

    auto [_, las] = ff[goals];
//    for (auto i: las) cout << i << " ";
    char  now = 'a';
    string ans = "";
    reverse(las.begin(), las.end());
    for (auto i: las) {
        for (int j = 0; j < i; j++) {
            ans += now;
        }
        now++;
    }
    reverse(ans.begin(), ans.end());
    cout << ans << endl;

}