#include<bits/stdc++.h>
#define rep(i, l, r) for(int i = (l); i <= (r); i ++)
#define pre(i, l, r) for(int i = (l); i >= (r); i --)
using namespace std;

const int N = 200009;
typedef long long LL;

inline int read() {
    int x = 0; char fs = 0, c = getchar();
    while(! isdigit(c)) fs |= (c == '-'), c = getchar();
    while(isdigit(c)) x = (x << 1) + (x << 3) + (c ^ 48), c = getchar();
    return fs ? -x : x;
}

int n;
int cnt[29], pc[29];
char s[1009];
string st[1009];

int main() {
    n = read();
    scanf("%s", s + 1);
    rep(i, 1, n) {
        rep(c, 1, 26) {
            int ct = 0;
            memset(cnt, 0, sizeof cnt);
            pre(j, i, 1) {
                int x = s[j] - 'a' + 1;
                if(x == c) break;
                cnt[x] ++;
                if(cnt[x] == 1) ct ++;
            }
            pc[c] = ct + 1;
        }
        rep(j, 1, i) {
            st[i] = st[i] + (char)('a' + pc[s[j] - 'a' + 1] - 1);
        }
    }
    sort(st + 1, st + n + 1);
    cout << st[n] << endl;
    return 0;
}