#include <bits/stdc++.h>
using namespace std;

const int maxn = 1e6 + 5;
char s[maxn];

vector<int> calc(const vector<int> &z) {
	const int n = z.size();
	int a = 1;
	while (a < n && z[a] == z[0])
		a += 1;

	if (a == n)
		return z;

	vector<vector<int>> b(a, {z[0]});
	for (int i = n - 1, p = a - 1, q = p; i >= a; --i, (--p < 0) && (p = q)) {
		i != n - 1 && z[i] != z[i + 1] && (q = p);
		b[p].push_back(z[i]);
	}

	vector<int> y(a);
	y[0] = 0;
	for (int i = 1; i < a; ++i)
		y[i] = (b[i] == b[i - 1]) ? y[i - 1] : i;

	auto x = calc(move(y));
	vector<int> res;
	for (int i = 0; i < a; ++i)
		res.insert(res.end(), b[x[i]].begin(), b[x[i]].end());
	return res;
}


int main() {
	scanf("%s", s);
	int n = strlen(s);
	vector<int> y(n);
	for (int i = 0; i < n; ++i)
		y[i] = s[i] - 'a';
	sort(y.begin(), y.end());

	auto x = calc(y);
	for (auto c : x)
		putchar(c + 'a');
	puts("");
	return 0;
}