#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;
}

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

    string s;
    cin >> s;

    string ans = "a";

    for (int i = 1; i <= n; i ++) {
        string str = s.substr (0, i);

        int cnt = 0;
        array <int, 26> ok {};
        array <int, 26> code {};
        for (int j = str.size () - 1; j >= 0; j --) {
            if (ok[str[j] - 'a']) {
                continue;
            }
            ok[str[j] - 'a'] = 1;
            code[str[j] - 'a'] = cnt;
            cnt ++;
        }

        for (auto &c : str) {
            c = char (code[c - 'a'] + 'a');
        }
        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;
}