int main() {
#ifndef LOCAL
ios_base::sync_with_stdio(false);cin.tie(NULL);
#define endl '\n'
#endif
size_t m;
string line;
while (cin >> m >> line) {
size_t i = 0;
string ans;
a = line;
tree.n = line.size();
memset(tree.t, -1, sizeof tree.t);
tree.build(1, 0, line.size() - 1);
set<char> seen;
memset(choosed, 0, sizeof choosed);
while (i < line.size()) {
size_t to = min(line.size() - 1, i + m - 1);
size_t best = tree.query(1, i, to);
/*
size_t best = to;
for (int j = to; j >= int(i); --j) {
if (line[j] < line[best])
best = j;
}*/
ans.push_back(line[best]);
seen.insert(line[best]);
choosed[best] = true;
i = best + 1;
if (line.size() - best <= m) {
break;
}
}
char last = *(--seen.end());
for (size_t i = 0; i < line.size(); ++i) {
if (!choosed[i] && line[i] < last)
ans.push_back(line[i]);
}
sort(ans.begin(), ans.end());
cout << ans << endl;
}
return 0;
}
