#include <bits/stdc++.h>

using namespace std;

const int N = 5043;
const int M = 12;

bool dp[N][(1 << M)];

typedef pair<int, int> pt;

#define x first
#define y second

vector<int> bits[(1 << M)];

int main()
{
	string s;
	cin >> s;
	int n = s.size();
	string ans;
	dp[0][0] = 1;
	vector<pt> cur;
	cur.push_back(make_pair(0, 0));
	int final_sz = n;
	int cur_len = 1;
	int cnt = 0;
	while((1 << cnt) < n)
		cnt++;
	cnt--;

	for(int i = 0; i < (1 << cnt); i++)
		for(int j = 0; j < cnt; j++)
			if ((i & (1 << j)) == 0)
				bits[i].push_back(j);
	while(final_sz > cur_len)
	{
		final_sz -= cur_len;
		cur_len *= 2;
	}
	while(ans.size() < final_sz)
	{
		char min_chr = 'z';
		for(int i = 0; i < cur.size(); i++)
		{
			auto x = cur[i];
			for(auto y : bits[x.y])
			{
				if (dp[x.x + (1 << y)][x.y ^ (1 << y)] == 0)
				{
					cur.push_back(make_pair(x.x + (1 << y), x.y ^ (1 << y)));
					dp[x.x + (1 << y)][x.y ^ (1 << y)] = 1;
				}
			}
			min_chr = min(min_chr, s[x.x]);
		}
		vector<pt> new_cur;
		ans.push_back(min_chr);
		for(auto x : cur)
			if (s[x.x] == min_chr)
			{
				dp[x.x + 1][x.y] = 1;
				new_cur.push_back(make_pair(x.x + 1, x.y));
			}
		cur = new_cur;
	}
	cout << ans << endl;
}