Problem: Given a string s, determine if it contains any palindrome of length exactly 100 as a subsequence. If it has any, print any one of them. If it doesn't have any, print a palindrome that is a subsequence of s and is as long as possible.

Input

The only line of the input contains one string s of length n (1 ≤ n ≤ 5·104) containing only lowercase English letters.

Output

If s contains a palindrome of length exactly 100 as a subsequence, print any palindrome of length 100 which is a subsequence of s. If s doesn't contain any palindromes of length exactly 100, print a palindrome that is a subsequence of s and is as long as possible.

If there exists multiple answers, you are allowed to print any of them.

Examples

Input

bbbabcbbb


Output

bbbcbbb


Input

rquwmzexectvnbanemsmdufrg


Output

rumenanemur

Note

A subsequence of a string is a string that can be derived from it by deleting some characters without changing the order of the remaining characters. A palindrome is a string that reads the same forward or backward.

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
string dp[50001];
int last[50001][26];
int main() {
  int i, j, k, n;
  string a;
  cin >> a;
  n = a.size();
  last[1][a[0] - 'a'] = 1;
  for (i = 2; i <= n; i++) {
    for (j = 0; j < 26; j++) {
      last[i][j] = last[i - 1][j];
    }
    last[i][a[i - 1] - 'a'] = i;
  }
  for (i = n; i > 0; i--) {
    for (j = n; j > i; j--) {
      k = last[j][a[i - 1] - 'a'];
      if (k > i) {
        if ((k - i) == 1 && dp[j].size() < 2) {
          dp[j] = string(2, a[i - 1]);
        } else if ((dp[k - 1].size() + 2) > dp[j].size()) {
          dp[j] = a[i - 1] + dp[k - 1] + a[i - 1];
          if (dp[j].size() >= 100) {
            if (dp[j].size() == 101) dp[j].erase(50, 1);
            cout << dp[j];
            return 0;
          }
        }
      }
    }
    dp[i] = a[i - 1];
  }
  cout << dp[n] << endl;
  return 0;
}