#include<bits/stdc++.h>
using namespace std;
using Int = long long;
//INSERT ABOVE HERE
signed main(){
  string s;
  cin>>s;
  map<char, Int> m;
  for(char c:s) m[c]++;
  Int cnt=0;
  char c;
  for(auto p:m){
    if(p.second==2) cnt++;
    else c=p.first;
  }
  if(cnt==6) cout<<c<<endl;
  else cout<<"Impossible"<<endl;
  
  return 0;
}