#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
using namespace std;
typedef long long ll;

int main(){
    string s;
    cin >> s;
    map<char , int> mp;
    for(auto i : s){
        mp[i]++;
    }
    if(mp.size() != 7){
        cout << "Impossible" << endl;
    }else{
        for(auto i : mp){
            if(i.second == 1){
                cout << i.first << endl;
                break;
            }
        }
    }
    return 0;
}