#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n;
  string s;
  cin >> s;
  n = s.length();
  string ans = "";
  long long i = 0, j = n - 1;
  while (j - i > 1) {
    if (s[i] == s[j]) {
      ans += s[i];
      i++;
      j--;
    } else if (s[i] == s[j - 1]) {
      ans += s[i];
      i++;
      j -= 2;
    } else if (s[i + 1] == s[j]) {
      ans += s[j];
      j--;
      i += 2;
    } else {
      i++;
      j--;
    }
  }
  cout << ans;
  if (i == j) {
    cout << s[i];
  } else {
    if (s[i] == s[j])
      cout << s[i] << " " << s[i] << '\n';
    else
      cout << s[i];
  }
  for (long long int i = 0; i < ans.length(); i++)
    cout << ans[ans.length() - 1 - i];
  return;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int TESTS = 1;
  while (TESTS--) {
    solve();
  }
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
  return 0;
}
