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

#define rep(i,n) for(int i=0;i<n;i++)

signed main(){
  string s;
  cin>>s;
  map<char,int>mp;
  rep(i,13){
    mp[s.at(i)]++;
  }
  int a=0;
  int b=0;
  char pai;
  for(auto e:mp){
    if(e.second==2) a++;
    if(e.second==1){
      b++;
      pai=e.first;  
    }
  }
  if(a==6 && b==1){
    cout<<pai<<endl;
  }
  else{
    cout<<"Impossible"<<endl;
  }
  return 0;
}