#include <iostream>
#include <algorithm>
#include <vector>
#include <string>

using namespace std;

int main() {
  string s;
  cin >> s;
  if (s.size() != 13) {
    cout << "Impossible" << endl;
    return 0;
  }
  sort(s.begin(), s.end());
  string t(s);
  t.erase(unique(t.begin(), t.end()), t.end());

  if (t.size() != 7) {
    cout << "Impossible" << endl;
    return 0;
  }

  char ans = 0;
  for (char c : s) {
    ans ^= c;
  }
  cout << ans << endl;
}