#include <bits/stdc++.h>
#define int long long

using namespace std;

inline int read () {
    int w = 1, s = 0; char ch = getchar ();
    for (; ! isdigit (ch); ch = getchar ()) if (ch == '-') w = -1;
    for (; isdigit (ch); ch = getchar ()) s = (s << 1) + (s << 3) + (ch ^ 48);
    return s * w;
}

const int N = 20;

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

    auto work = [&] (const string s) {
        vector <int> last (N, -1);
        const int n = s.size ();
        for (int i = n - 1; i >= 0; i --) {
            int t = s[i] - 'a';
            if (last[t] == -1) {
                last[t] = i;
            }
        }
        vector <char> c (N);
        for (int i = 0; i < N; i ++) {
            if (last[i] != -1) {
                set <char> st;
                for (int j = last[i] + 1; j < n; j ++) {
                    st.insert (s[j]);
                }
                c[i] = char ('a' + st.size ());
            }
        }

        string ans;
        for (auto cc : s) {
            ans += c[cc - 'a'];
        }
        return ans;
    };

    string ans = "A";
    for (int i = 0; i < n; i ++) {
        auto str = work (s.substr (0, i + 1));
        ans = max (ans, str);
    }

    cout << ans << '\n';
} 

signed main () {
	ios::sync_with_stdio (false);
    cin.tie (nullptr);
    
	int T = 1; 
	// cin >> T;
	// T = read ();

	while (T --) {
		solve ();
	}
	
	return 0;
}