#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<utility>
#define REP(i,n) for(int i = 0;i < (n);i++)
#define pb push_back
using namespace std;
const int INF = 1e9;
typedef long long ll;

int main(){
  string s;
  cin >> s;
  int a[26]={0};
  REP(i,s.size()){
    a[s[i]%26]++;
  }
  int sum = 0;
  char t;
  for(int i = 19;i < 19+26;i++){
    if(a[i%26] == 2){
      sum++;
      //t = 97+(i-19);
    }
    else if(a[i%26] == 1){
      t = 97+(i-19);
    }
  }
  if(sum == 6)
    cout << t << endl;
  else
    cout << "Impossible" << endl;
  return 0;
}