#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;

int main(){
    int table[26]={},count=-1;
    char out;
    string str;
    cin >> str;
    for(int i=0;str[i]!='\0';i++){
        table[str[i]-'a']++;
    }
    for(int i=0;i<26;i++){
        if(!(table[i]==2||table[i]==0)){
            if(table[i]>2)  count++;
            out = 'a' + i;
            count++;
        }
    }
    if(count)   cout << "Impossible" << endl;
    else    cout << out << endl;
}