#include <bits/stdc++.h>

#define N 1010
using namespace std;

string s;
string t;
int n, r, mx;
bool v[300];
int a[N], sum, p;
int main() {
    cin>>n; r = n;
    cin>>s; s = " " + s + " ";
    for(int i = 1;i <= r; i++) {
        memset(v, 0, sizeof(v));
        sum = 0; mx = 0; v[s[i]] = 1; s[r + 1] = s[i];
        for(int j = i + 1; j <= r + 1; j++) {
            if(v[s[j] ]== 0) sum++, v[s[j]] = 1;
            if(s[j] == s[i]) {
                a[j] = sum; sum = 0;
                memset(v, 0, sizeof(v));
                if(a[mx] <= a[j]) mx  = j;
                v[s[j]] = 1;
            }
            
        }
        t += char('a' + a[mx]); 
        r = mx - 1;
    }
    cout<<t<<endl;
    return 0;
}