//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC optimize("Ofast")

#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <iomanip>

using namespace std;

typedef long long ll;

#define pb push_back
#define pii pair<int, int>
#define mp make_pair
#define F first
#define S second
#define sorted(a) sort(a.begin(), a.end())
#define pop pop_back

int len;

bool check(string& s, vector <int>& a, int& mid) {
    for (int i = len - 1; i >= (mid * (mid + 1)) / 2 - 1; --i) {
        if (a[i] < mid) {
            continue;
        }
        else {
            bool ok = true;
            int part = mid;
            int j = i;
            while (part > 1) {
                j -= part;
                if (a[j] < part - 1) {
                    ok = false;
                    break;
                }
                part -= 1;
            }
            if (ok) return true;
        }
    }
    return false;
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    string s;
    cin >> s;
    len = (int)s.size();
    vector <int> a(len);
    a[0] = 1;
    for (int i = 1; i < len; ++i) {
        a[i] = ((s[i] == s[i - 1]) ? a[i - 1] + 1 : 1);
    }
    int left = 0, right = (int)sqrt(len) + 2;
    while (right - left > 1) {
        int mid = (right + left) / 2;
        if (check(s, a, mid)) {
            left = mid;
        }
        else {
            right = mid;
        }
    }
    string ans;
    for (int i = len - 1; i >= (left * (left + 1)) / 2 - 1; --i) {
        if (a[i] < left) {
            continue;
        }
        else {
            bool ok = true;
            int part = left;
            int j = i;
            string ss = "";
            while (part > 1) {
                for (int k = 0; k < part; ++k) {
                    ss += s[j];
                }
                j -= part;
                if (a[j] < part - 1) {
                    ok = false;
                    break;
                }
                part -= 1;
            }
            if (ok) {
                ans = ss + s[j];
                reverse(ans.begin(), ans.end());
                break;
            }
        }
    }
    cout << ans;
    return 0;
}