#include <bits/stdc++.h>
#pragma GCC optimize(3)
using namespace std;
using i64 = long long;
using u64 = unsigned i64;

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    constexpr int S = 20;

    int T = 1;// cin >> T;
    while(T--){
        int n; cin >> n;
        string str; cin >> str;
        array<vector<int>, S> pre;
        pre.fill(vector<int>(n+1, 0));
        array<int, S+1> last; last.fill(0); last[S] = n;
        for(int j=0; j<S; j++){
            for(int i=1, cnt=0; i<=n; i++){
                pre[j][i] = pre[j][i-1] + (str[i-1]==j+'a');
            }
            last[j] = lower_bound(pre[j].begin(), pre[j].end(), pre[j][n]) - pre[j].begin();
        }

        // using State = vector<short>;
        constexpr u64 MASK = 0x3FF;
        // struct State{
        //     array<int, S> v;
        //     void set(int i, int val){
        //         v[i] = val;
        //     }
        //     int get(int i){
        //         return v[i];
        //     }
        //     inline bool operator>(const State&o)const{
        //         return v > o.v;
        //     }
        // };
        struct State{
            // 10 * 20 == 200
            u64 v[4] = {0, 0, 0, 0};
            inline void set_v(int i, int j, u64 val){
                v[i] &= ~(MASK << (50 - j*10));
                v[i] |= (val & MASK) << (50 - j*10);
            }
            inline void set(int i, u64 val){
                // if(val == 0) return;
                set_v(i/6, i%6, val);
            }
            inline u64 get_v(int i, int j){
                return (v[i] >> (50 - j*10)) & MASK;
            }
            inline u64 get(int i){
                return get_v(i/6, i%6);
            }
            inline bool operator<(const State&o)const{
                if(v[0] != o.v[0]) return v[0] > o.v[0];
                if(v[1] != o.v[1]) return v[1] > o.v[1];
                if(v[2] != o.v[2]) return v[2] > o.v[2];
                return v[3] > o.v[3];
            }
        };

        auto lowbit = [](auto x){
            return x&-x;
        };

        constexpr int N = 1<<S;
        vector<pair<State, int>> dp(N);
        dp[0] = {{}, 0};
        for(int s=0; s<N; s++){
            int cnt = __builtin_popcount(s);
            auto&[state, pos] = dp[s];
            auto mx=1<<S;
            for(int j=s^(N-1); j; j-=lowbit(j)){
                auto k = lowbit(j);
                if(last[__lg(mx)] > last[__lg(k)]){
                    mx = k;
                }
            }
            for(int j=s^(N-1); j; j-=lowbit(j)){
                auto k = lowbit(j);
                auto bound = last[__lg(mx)];
                auto ki = __lg(k);
                auto new_dp = make_pair(state, (int)(lower_bound(pre[ki].begin(), pre[ki].end(), pre[ki][bound]) - pre[ki].begin()));
                new_dp.first.set(cnt, pre[ki][bound] - pre[ki][pos]);
                // cout << "bound[" << ki <<  "]: " << bound << " " << pos << " => " << pre[ki][bound] - pre[ki][pos] << endl;
                if(new_dp < dp[s|k]){
                    dp[s|k] = move(new_dp);
                }
            }
        }
        char c = 'a' + S - 1;
        for(int i=0; i<S; i++){
            int x = dp[N-1].first.get(i);
            // cout << x << endl;
            while(x--) cout << c;
            c--;
        }
        cout << endl;
    }

    return 0;
}