#include <bits/stdc++.h>

using namespace std;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;
using f64 = double;

void solve() {
    int n;
    cin >> n;

    string s;
    cin >> s;

    auto t = s;
    sort(t.begin(), t.end());
    t.erase(unique(t.begin(), t.end()), t.end());
    for (auto &c : s) {
        c = 'a' + lower_bound(t.begin(), t.end(), c) - t.begin();
    }

    int w = t.size();
    vector<int> last(w, -1);
    for (int i = 0; i < n; i++) {
        last[s[i] - 'a'] = i + 1;
    }

    vector<int> f(n + 1);
    for (int i = 0; i <= n; i++) {
        for (int j = 0; j < w; j++) {
            if (last[j] > i) {
                f[i] |= 1 << j;
            }
        }
    }

    string ans;
    for (int i = 1, j = 0, c = 'a' + w - 1; i <= n; i++) {
        if (f[i] != f[j]) {
            for (int k = j; k < i; k++) {
                if ((f[j] - f[i]) == (1 << (s[k] - 'a'))) {
                    ans.push_back(c);
                }
            }
            j = i;
            c--;
        }
    }
    cout << ans << "\n";
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(16);

    int t = 1;

    while (t--) {
        solve();
    }

    return 0;
}