#include <bits/stdc++.h>
#pragma comment(linker, "/stack:256000000")
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
mt19937_64 gen(time(NULL));
ll const maxn = 3e6 + 6;
unordered_map<char, ll> mapa;
bool used[maxn];
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string s;
  cin >> s;
  ll n = s.size();
  fill_n(used, maxn, 0);
  ll l = 0, r = n - 1;
  while (r >= l) {
    if (s[r] == s[l]) {
      used[r] = 1;
      used[l] = 1;
      r--;
      l++;
    } else {
      if (s[l] == s[r - 1]) {
        used[r - 1] = 1;
        used[l] = 1;
        r -= 2;
        l++;
      } else if (s[l + 1] == s[r]) {
        used[l + 1] = 1;
        used[r] = 1;
        r--;
        l += 2;
      } else if (s[l + 1] == s[r - 1]) {
        used[r - 1] = 1;
        used[l + 1] = 1;
        r -= 2;
        l += 2;
      }
    }
  }
  string ss = "";
  for (ll i = 0; i < n; i++) {
    if (used[i]) ss += s[i];
  }
  if (ss.size() < (n) / 2) {
    cout << "IMPOSSIBLE";
  } else {
    cout << ss;
  }
  return 0;
}
