#include <bits/stdc++.h>

using namespace std;


int main()
{
  string temp = "abcdefghijklm";

  string S;
  cin >> S;
  int sum[26] = {};
  for(auto &c : S) sum[c - 'a']++;

  bool flag = false;
  for(int i = 0; i < temp.size(); i++) {
    sum[i]++;
    int one = 0, two = 0, other = 0;
    for(int j = 0; j < temp.size(); j++) {
      if(sum[j] == 1) ++one;
      else if(sum[j] == 2) ++two;
      else ++other;
    }
    if(other == 0 && two == 1) {
      cout << temp[i] << endl;
      flag = true;
    }
    sum[i]--;
  }
  if(!flag) cout << "Impossible" << endl;
}