#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000 * 1000 * 1000 + 7;
const long long MOD2 = 998244353;
const long long N = 1000 * 1000 + 5;
const double PI = 3.14159265;
long long power(long long x, long long y, long long p) {
  long long res = 1;
  x %= p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string s;
  cin >> s;
  if (s.size() < 4)
    cout << s[0];
  else {
    std::vector<char> v;
    int n = s.size();
    int j = n - 1;
    int i = 0;
    while ((j - i) > 2) {
      if (s[i] == s[j]) {
        v.push_back(s[i]);
        i++;
        j--;
      } else if (s[i] == s[j - 1]) {
        v.push_back(s[i]);
        i++;
        j--;
        j--;
      } else if (s[i + 1] == s[j]) {
        v.push_back(s[i + 1]);
        i += 2;
        j--;
      } else {
        v.push_back(s[i + 1]);
        i += 2;
        j -= 2;
      }
    }
    stack<char> q;
    for (auto x : v) {
      q.push(x);
      cout << x;
    }
    cout << s[i];
    while (!q.empty()) {
      cout << q.top();
      q.pop();
    }
  }
  return 0;
}
