#include<bits/stdc++.h>
#define MAX_N 100001
#define INF_INT 2147483647
#define INF_LL 9223372036854775807
#define REP(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
typedef long long int ll;
typedef pair<ll,ll> P;

bool isPrime(int n){
  if(n == 2)
    return true;
  if(n % 2 == 0)
    return false;
  int sqr = sqrt(n)+1;
  for(int i=3;i<=sqr;i++){
    if(n % i == 0)
      return false;
  }
  return true;
}

int main()
{
  int n;
  char d;
  bool f=false;
  string S;
  map<char,int> mp;
  cin >> S;
  REP(i,S.size()){
    mp[S[i]]++;
  }
  REP(i,S.size()){
    if(mp[S[i]] >= 3){
      cout << "Impossible" << endl;
      return 0;
    }
    if(mp[S[i]] == 1){
      if(f){
        cout << "Impossible" << endl;
        return 0;
      }
      f = true;
      d = S[i];
    }
  }
  cout << d << endl;
  return 0;
}