#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);(i)++)

string s;
int main(){
  cin >> s;
  for(char add = 'a';add <= 'z';add++){
    map<char, int> mp;
    string t = s;
    t.push_back(add);
    for (char c : t)
      mp[c]++;
    bool ok = true;
 
    for(auto p : mp){
      ok = ok && p.second == 2;
    }

    if(ok){
      cout << add << endl;
      return 0;
    }
  }

  cout << "Impossible" << endl;
  return 0;
}