#include <bits/stdc++.h>
using namespace std;
bool dp[5001][5001];
vector<int> bits[10001];
vector<pair<int, int> > cur;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s, ans;
  cin >> s;
  int n = s.size();
  dp[0][0] = 1;
  cur.push_back(make_pair(0, 0));
  int cnt = 0;
  while ((1 << cnt) < n) cnt++;
  cnt--;
  ans = "";
  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);
  int l = 1;
  int m = n;
  while (m > l) {
    m -= l;
    l *= 2;
  }
  while (ans.size() < m) {
    char min_chr = 'z';
    for (int i = 0; i < cur.size(); i++) {
      auto x = cur[i];
      for (auto j : bits[x.second]) {
        if (dp[x.first + (1 << j)][x.second | (1 << j)] == 0) {
          cur.push_back(make_pair(x.first + (1 << j), x.second | (1 << j)));
          dp[x.first + (1 << j)][x.second | (1 << j)] = 1;
        }
      }
      min_chr = min(min_chr, s[x.first]);
    }
    ans.push_back(min_chr);
    vector<pair<int, int> > tmp;
    for (auto x : cur) {
      if (s[x.first] == min_chr) {
        dp[x.first + 1][x.second] = 1;
        tmp.push_back(make_pair(x.first + 1, x.second));
      }
    }
    cur = tmp;
  }
  cout << ans << '\n';
  return 0;
}
