#include <bits/stdc++.h>
using namespace std;
const long long inf = 2e18;
const long long mod = 1e9 + 7;
const long double pi = 3.141592653589793238462643383279502884;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
void print(long long a[], long long n) {
  for (long long i = 0; i < n; i++) {
    cout << a[i] << " ";
  }
  cout << "\n";
}
long long power(long long x, long long y) {
  if (y <= 0) return 1;
  long long ans = 1;
  x %= mod;
  while (y) {
    if (y & 1) ans = (x * ans) % mod;
    x = (x * x) % mod;
    y >>= 1;
  }
  return ans;
}
long long modInverse(long long n) { return power(n, mod - 2); }
inline long long mul(long long a, long long b) { return (a * b) % mod; }
inline long long sub(long long a, long long b) {
  long long c = (a - b);
  if (c < 0) c += mod;
  return c;
}
inline long long add(long long a, long long b) {
  long long c = (a + b);
  if (c >= mod) c -= mod;
  return c;
}
inline long long divi(long long a, long long b) {
  return mul(a, modInverse(b));
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  cout << fixed << setprecision(10);
  string s;
  cin >> s;
  int fa = 0, fb = 0, fc = 0;
  for (int i = 0; i < s.length(); i++) {
    fa += (s[i] == 'a');
    fb += (s[i] == 'b');
    fc += (s[i] == 'c');
  }
  if (fa >= (s.length() / 2)) {
    for (int i = 0; i < s.length(); i++) {
      if (s[i] == 'a') cout << s[i];
    }
    return 0;
  }
  if (fb >= (s.length() / 2)) {
    for (int i = 0; i < s.length(); i++) {
      if (s[i] == 'b') cout << s[i];
    }
    return 0;
  }
  if (fc >= (s.length() / 2)) {
    for (int i = 0; i < s.length(); i++) {
      if (s[i] == 'c') cout << s[i];
    }
    return 0;
  }
  int i = 0, j = s.size() - 1;
  string ans = "";
  while (j - i >= 3) {
    if (s[i] == s[j]) {
      ans += s[i];
      i++, j--;
    } else if (s[i] == s[j - 1]) {
      ans += s[i];
      i++, j -= 2;
    } else if (s[i + 1] == s[j]) {
      ans += s[i + 1];
      i += 2, j--;
    } else if (s[i + 1] == s[j - 1]) {
      ans += s[i + 1];
      i += 2, j -= 2;
    }
  }
  string ans1 = ans;
  if (j >= i) ans1 += s[i];
  reverse(ans.begin(), ans.end());
  ans1 += ans;
  if (ans1.size() >= (s.size() / 2))
    cout << ans1 << "\n";
  else
    cout << "IMPOSSIBLE\n";
  return 0;
}
