#include <bits/stdc++.h>
using namespace std;
void solve() {
  string s;
  cin >> s;
  long long n = s.length();
  long long req = n / 2;
  long long l = 0, r = n - 1;
  vector<char> ans;
  char extra;
  bool flag = 0;
  while (l < r) {
    if (req <= 0) break;
    if (req == 1) {
      extra = s[l];
      flag = 1;
      req--;
      break;
    }
    if (s[l] == s[r]) {
      ans.push_back(s[l]);
      l++;
      r--;
    } else if (l + 1 < r && s[l + 1] == s[r]) {
      ans.push_back(s[r]);
      l += 2;
      r--;
    } else if (l < r - 1 && s[r - 1] == s[l]) {
      ans.push_back(s[l]);
      l++;
      r -= 2;
    } else if (l + 1 < r - 1 && s[l + 1] == s[r - 1]) {
      ans.push_back(s[r - 1]);
      l += 2;
      r -= 2;
    }
    req -= 2;
  }
  if (req > 0) {
    cout << "IMPOSSIBLE\n";
    return;
  }
  for (long long i = 0; i < ((long long)ans.size()); i++) cout << ans[i];
  if (flag) cout << extra;
  for (long long i = ((long long)ans.size() - 1); i >= 0; i--) cout << ans[i];
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) solve();
}
