#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Graph = vector<vector<int>>;
int main() {
  string S; cin >> S;
  map<char,int> dic;
  for(int i =0; i < S.size(); i++) {
    dic[S[i]]++;
  }
  int count = 0;
  char ans = 'e';
  for (const auto& [key, value] : dic) {
    if(value == 2) {
      count++;
    }else {
      ans = key;
    }
  }
  if(count == 6) {
    cout << ans << endl;
  }else {
    cout << "Impossible" << endl;
  }
}