#include <bits/stdc++.h>

using namespace std;

int main(void) {
  string s;
  cin >> s;

  map<char, int> mp;
  for (int i = 0; i < s.size(); i++) {
    mp[s[i]]++;
  }

  int one = 0, two = 0;
  string ans = "Impossible";
  for (auto i : mp) {
    one += i.second == 1;
    two += i.second == 2;
    if (i.second == 1) ans = i.first;
  }
  cout << (one == 1 && two == 6 ? ans : "Impossible") << endl;

  return 0;
}