#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1e9 + 7, oo = 1e9;
const ll loo = 1e18;
ll modpow(ll a, ll b) {
  ll ans = 1;
  for (; b; b /= 2, a = a * a % mod)
    if (b & 1) ans = (ans * a) % mod;
  return ans;
}
ll gcd(ll a, ll b) {
  while (a) b %= a, swap(a, b);
  return b;
}
const bool DEBUG = 1;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  int n = s.length();
  int pre[n][3];
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < 3; j++) pre[i][j] = i ? pre[i - 1][j] : -oo;
    pre[i][s[i] - 'a'] = i;
  }
  stringstream ss;
  int i = 0, j = n - 1;
  int cnt = 0;
  while (i <= j) {
    int jj = pre[j][s[i] - 'a'];
    if (jj - i >= 0 && (jj - i - 1) / 2 + cnt + (1 + (i != jj)) >= n / 2) {
      j = jj - 1;
      cnt += 1 + (i != jj);
      ss << s[i];
    }
    i++;
  }
  string t = ss.str();
  string r;
  if (cnt & 1)
    r = t.substr(0, t.length() - 1);
  else
    r = s.substr(0, t.length());
  reverse(r.begin(), r.end());
  cout << t << r << endl;
  return 0;
}
