#include <bits/stdc++.h>
using namespace std;
string second, pre, suf;
int main() {
  ios::sync_with_stdio(false);
  cin >> second;
  int n = (int)second.size();
  int l = 0;
  int r = n - 1;
  while (l + 1 < r - 1) {
    if (second[l] == second[r]) {
      pre += second[l];
      suf += second[r];
    } else {
      if (second[l] == second[r - 1])
        pre += second[l], suf += second[r - 1];
      else if (second[l + 1] == second[r])
        pre += second[l + 1], suf += second[r];
      else
        pre += second[l + 1], suf += second[r - 1];
    }
    l += 2;
    r -= 2;
  }
  if (n % 4 != 0) {
    pre += second[n / 2];
  }
  reverse(suf.begin(), suf.end());
  pre = pre + suf;
  cout << pre;
  return 0;
}
