#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main() {
  string s;
  cin >> s;
  map<char, int> mp;
  for (int i = 0; i < 13; ++i) mp[s[i]]++;

  if (mp.size() != 7) {
    puts("Impossible");
  } else {
    int ans = -1;
    for (auto const &e: mp) {
      if (e.second == 1) {
	if (ans != -1) {
	  puts("Impossible");
	  return 0;
	}
	ans = e.first;
      }
    }
    cout << (char)ans << '\n';
  }
  return 0;
}