#include <bits/stdc++.h>

using namespace std;

const string chars = "abcdefghijklm";

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  string S;
  cin >> S;
  map<char, int> mp;
  int cnt = 0;
  for (char s : S) {
    mp[s]++;
    if (mp[s] > 1) cnt++;
  }
  if (cnt >= 2) {
    cout << "Impossible" << '\n';
    return 0;
  }
  for (char s : chars) {
    if (mp[s] == 0 && cnt == 1) {
      cout << s << '\n';
    }
    if (mp[s] == 1 && cnt == 0) {
      cout << s << '\n';
    }
  }
  return 0;
}