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

int main() {
  string S;
  cin >> S;
  vector<int> A(26);
  int pair = 0, ans;

  for (char c : S) A.at(c - 'a')++;
  for (int i = 0; i < 26; i++) {
    if (A.at(i) == 2)
      pair++;
    else if (A.at(i) == 1)
      ans = i;
  }
  
  if (pair == 6)
    cout << (char)(ans + 'a') << endl;
  else
    cout << "Impossible" << endl;
}