#include <bits/stdc++.h>
using namespace std;
const int N = 1e6;
const int oo = 1e9 + 5;
int du[] = {-1, 0, 0, 1};
int dv[] = {0, -1, 1, 0};
const long long mod = 1e9 + 7;
long long sqr(long long x) { return x * x; }
string s;
int n;
bool ok[N + 5];
int main() {
  ios_base::sync_with_stdio(false);
  cin >> s;
  n = s.length();
  s = " " + s;
  for (int l = 1, r = n; l <= r; l += 2, r -= 2) {
    if (s[l] == s[r]) {
      ok[l] = true;
      ok[r] = true;
      if (s[l + 1] == s[r - 1]) {
        ok[l + 1] = true;
        ok[r - 1] = true;
      }
    } else if (s[l] == s[r - 1]) {
      ok[l] = true;
      ok[r - 1] = true;
    } else if (s[l + 1] == s[r]) {
      ok[l + 1] = true;
      ok[r] = true;
    } else {
      ok[l + 1] = true;
      ok[r - 1] = true;
    }
  }
  int dem = 0;
  for (int i = 1; i <= n; i++) dem += ok[i];
  if (dem < n / 2)
    cout << "IMPOSSIBLE";
  else
    for (int i = 1; i <= n; i++)
      if (ok[i]) cout << s[i];
  return 0;
}
