#include <iostream>
using namespace std;
int alp[27];
int main(){
    string s;cin>>s;
    for(int i = 0; s.size() > i; i++){
        alp[s[i]-'a']++;
    }
    char ans = 'A';
    for(int i = 0; 27 > i; i++){
        if(alp[i]>2){
            cout << "Impossible" << endl;
            return 0;
        }
        if(alp[i]==1){
            if(ans != 'A'){
                cout << "Impossible" << endl;
                return 0;
            }
            ans = i+'a';
        }
    }
    cout << ans << endl;
}