#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)

template<typename S,typename T,typename U>void flagfunc1(S &flag,T A,U B){flag?cout<<A<<'\n':cout<<B<<'\n';}

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  string s;cin>>s;
  map<char,int> map;
  rep(i,13)map[s[i]]++;
  char hoge;
  bool flag=false;
  for(auto itr=map.begin();itr!=map.end();++itr){
   //cout<<"key="<<itr->first<<",val="<<itr->second<<"\n";
   if(!flag&&itr->second==1){
     hoge=itr->first;
     flag=true;
   }else if((flag&&itr->second==1)||itr->second>2){
     break;
   }
  }
  flagfunc1(flag,hoge,"Impossible");
}