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

typedef pair<int, int> pi;
typedef long long LL;

int main() {
    ios::sync_with_stdio(0), cin.tie(0);
    int n; cin >> n;
    string s; cin >> s;
    s = " " + s;
    V<V<int>> lst_ocur(n + 2, V<int>(20));
    V<int> lst_elim(1 << 20);
    V<V<int>> sum(n + 2, V<int>(20));

    int hav = 0;
    for(int i = 1; i <= n + 1; i++) {
        if(i != n + 1) hav |= 1 << (s[i] - 'a');
        lst_ocur[i] = lst_ocur[i - 1];
        if(i != n + 1) lst_ocur[i][s[i] - 'a'] = i;
        sum[i] = sum[i - 1];
        if(i != n + 1) sum[i][s[i] - 'a']++;
    }
    
    for(int i = 0; i < (1 << 20); i++) {
        int mn = 1e9;   
        for(int j = 0; j < 20; j++) {
            if(i >> j & 1) mn = min(mn, lst_ocur[n][j]);
        }
        lst_elim[i] = mn;
    }
    lst_elim[0] = n + 1;

    V<int> ans(20);
    V<V<int>> able(1 << 20, V<int>(20));
    V<V<int>> dp(1 << 20, V<int>(20));
    for(int i = 20; i >= 1; i--) {
        if(i == __popcount(hav)) {
            able[hav][0] = 1;
        }

        //cal dp, clear able

        int mx = 0;
        for(int mask = 0; mask < 1 << 20; mask++) {
            if(__popcount(mask) != i) continue;
            for(int j = 0; j < 20; j++) {
                if(!able[mask][j]) continue;
                for(int k = 0; k < 20; k++) if(mask >> k & 1) {
                    int lst = lst_elim[mask];
                    lst = lst_ocur[lst][j];
                    if(mask == hav) lst = 0;

                    int to = lst_ocur[lst_elim[mask - (1 << k)]][k];
                    int dp_val = sum[to][k] - sum[lst][k];
                    mx = max(dp_val, mx);
                    dp[mask - (1 << k)][k] = max(dp_val, dp[mask - (1 << k)][k]);
                    able[mask - (1 << k)][k] = 1;
                }
                able[mask][j] = 0;
            }
        }
        
        //upd ans + upd able
        ans[i - 1] = mx;
        for(int mask = 0; mask < 1 << 20; mask++) {
            if(__popcount(mask) != i - 1) continue;
            for(int j = 0; j < 20; j++) {
                if(!able[mask][j]) continue;
                if(dp[mask][j] != mx) able[mask][j] = 0;
            }
        }
    }


    // for(int i = 19; i >= 0; i++) cout << ans[i] << " ";
    for(int i = 19; i >= 0; i--) {
        for(int j = 1; j <= ans[i]; j++) cout << (char)('a' + i);
    }
    cout << endl;
}