#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);(i)++)

string s;
int main(){
  cin >> s;
  map<char,int> mp;
  for(char c : s){
    mp[c]++;
  }
  vector<pair<char,int>> ans;
  for(auto p : mp){
    if(p.second == 1)
      ans.push_back(p);
  }
  if(ans.size() == 1){
    cout << ans[0].first << endl;
  }
  else{
    cout << "Impossible" << endl;
  }
}