#include <iostream>
using namespace std;
 
int main(){
    string S;
    cin >> S;
    char alphabets[26] = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
    int in_alphabets[26] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    for(int i = 0; i < 13; i++){
        for(int j = 0; j < 26; j++){
            if(S[i] == alphabets[j]){
                in_alphabets[j]++;
                break;
            }
        }
    }
    int one = -1;
    for(int i = 0; i < 26; i++){
        if((in_alphabets[i] == 1 && one != -1) || in_alphabets[i] >= 3){
            cout << "Impossible" << endl;
            return 0;
        }else if(in_alphabets[i] == 1){
            one = i;
        }
    }
    cout << alphabets[one] << endl;
}