#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  char ex = '\0';
  cin >> s;
  string str = "";
  if (s.length() <= 3) {
    cout << s[0] << endl;
  } else {
    int i = 0, j = s.length() - 1;
    while (i < j) {
      if (i == j) {
        ex = s[i];
        break;
      }
      if (s[i] == s[j]) {
        str.push_back(s[i]);
      } else if (j - 1 > i && s[i] == s[j - 1]) {
        str.push_back(s[i]);
        j = j - 1;
      } else if (i + 1 < j && s[i + 1] == s[j]) {
        str.push_back(s[j]);
        i = i + 1;
      } else {
        if (i + 1 < j - 1) {
          str.push_back(s[i + 1]);
          ++i;
          --j;
        } else {
          ex = s[i + 1];
          ++i;
          --j;
        }
      }
      ++i;
      --j;
      if (str.length() * 2 >= (s.length() / 2)) break;
    }
    string strr = str;
    reverse(str.begin(), str.end());
    if (ex == '\0') {
      cout << strr << str << endl;
    } else
      cout << strr << ex << str << endl;
  }
}
