#define F_C
#include <bits/stdc++.h>
#define rep(i, l, r) for(int i = l; i < r; i++)
#define hrp(i, l, r) for(int i = l; i <= r; i++)
#define rev(i, r, l) for(int i = r; i >= l; i--)
#define int ll
using namespace std;
typedef long long ll;
template<typename tn = int> tn next(void) { tn k; cin>>k; return k; }
#ifndef LOCAL
#define D(...) 0
#endif
const int C = 20, U = 1200;
pair<int, int> dp[(1<<C)+U];
int maxap[C+5], from[(1<<C)+U], pos[(1<<C)+U];
int sum[C+5][U], lst[C+5][U];
signed main(void)
{
    #ifdef LOCAL
//	freopen("C:\\Users\\Loxil\\Desktop\\IN.txt", "r", stdin);
//	freopen("C:\\Users\\Loxil\\Desktop\\OUT.txt", "w", stdout);
    #endif
    
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int n, resmsk = 0;
    string str;
    cin>>n>>str;
    str = '$'+str;

    hrp(i, 1, n) rep(j, 0, C) sum[j][i] = sum[j][i-1]+(j == str[i]-'a'), lst[j][i] = j == str[i]-'a' ? i : lst[j][i-1];
    rep(i, 0, 20) if (sum[i][n]) resmsk |= 1<<i;

    rep(msk, 0, 1<<C)
    {
        int minn = n+1;
        rep(i, 0, 20) if (!((msk>>i)&1) && lst[i][n]) minn = min(minn, lst[i][n]);
        pos[msk] = minn-1;
    }

    rep(msk, 0, 1<<C)
    {
        int cnt1 = __builtin_popcount(msk); 
        if ((msk|resmsk) != resmsk || dp[msk].first < maxap[cnt1]) continue;
        rep(i, 0, 20) if (!(msk&(1<<i)) && resmsk&(1<<i))
        {
            int cntn = sum[i][pos[msk|(1<<i)]]-sum[i][dp[msk].second];
            if (cntn > maxap[cnt1+1]) dp[msk|(1<<i)] = {cntn, lst[i][pos[msk|(1<<i)]]}, maxap[cnt1+1] = cntn, from[msk|(1<<i)] = i;
            else if (cntn == maxap[cnt1+1] && (!dp[msk|(1<<i)].second || dp[msk|(1<<i)].second > lst[i][pos[msk|(1<<i)]])) dp[msk|(1<<i)] = {cntn, lst[i][pos[msk|(1<<i)]]}, from[msk|(1<<i)] = i;
        }
    }

    string ans;
    char ch = 'a';
    while(resmsk)
    {
        int f = from[resmsk], cnt = maxap[__builtin_popcount(resmsk)];
        rep(i, 0, cnt) ans += ch;
        resmsk ^= 1<<f;
        ch++;
    }

    reverse(ans.begin(), ans.end());
    cout<<ans<<endl;
    
    return 0;
}
/*
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 */