#include <bits/stdc++.h>
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int grand(int x) { return uniform_int_distribution<int>(0, x - 1)(rng); }
long long gcd(long long x, long long y) {
  if (x < y) return gcd(y, x);
  if (y == 0) return x;
  return gcd(y, x % y);
}
const long long mod = 1e9 + 7;
long long modexp(long long x, long long ex) {
  long long ans = 1ll;
  while (ex > 0) {
    if (ex & 1ll) ans = (ans * x) % mod;
    ex >>= 1ll;
    x = (x * x) % mod;
  }
  return ans;
}
const long long maxn = 1e6 + 7;
const long long inf = 1e9 + 7;
int32_t main() {
  {
    ios ::sync_with_stdio(false);
    cin.tie(0);
  };
  string s;
  cin >> s;
  string t;
  for (long long j = 0, k = s.size() - j - 1; j <= k;) {
    if (k - j + 1 <= 3) {
      cout << t << s[j];
      reverse((t).begin(), (t).end());
      cout << t << "\n";
      return 0;
    }
    if (s[j] == s[k] || s[j] == s[k - 1]) {
      t.push_back(s[j]);
    } else {
      t.push_back(s[j + 1]);
    }
    j++;
    j++;
    k--;
    k--;
  }
  cout << t;
  reverse((t).begin(), (t).end());
  cout << t << "\n";
}
