#include <bits/stdc++.h>
using namespace std;
long long int MOD = 998244353;
long long int powmod(long long int a, long long int b) {
  long long int res = 1;
  while (b) {
    if (b % 2 == 1) res = (res * a) % MOD;
    b /= 2;
    a = (a * a) % MOD;
  }
  return res;
}
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string s;
  cin >> s;
  vector<char> str;
  int l = 0, r = s.length() - 1;
  while ((r - l) > 2) {
    if (s[l] == s[r]) {
      str.push_back(s[l]);
    } else if (s[l] == s[r - 1]) {
      str.push_back(s[l]);
    } else if (s[l + 1] == s[r]) {
      str.push_back(s[l + 1]);
    } else {
      str.push_back(s[l + 1]);
    }
    l += 2;
    r -= 2;
  }
  int n = s.length();
  n /= 2;
  int cnt = 0;
  if (r - l >= 0) cnt++;
  if (str.size() * 2 + cnt < n) {
    cout << "IMPOSSIBLE\n";
    return 0;
  }
  n = str.size();
  for (auto x : str) cout << x;
  if (cnt) cout << s[l];
  for (int i = n - 1; i >= 0; i--) cout << str[i];
}
