#include <bits/stdc++.h>
using namespace std;

int main(){
    string S;
    cin >> S;
    vector<int> freq(26, 0);
    for (char c : S){
        ++freq[c - 'a'];
    }
    int c = 0;
    char t = '*';
    for (int i = 0; i < 26; ++i){
        if (freq[i] == 1) {
            ++c;
            t = 'a' + i;
        }else if (freq[i] >= 3){
            cout << "Impossible" << endl;
            return 0;
        };
    }
    if (c > 1){
        cout << "Impossible" << endl;
    }else{
        cout << t << endl;
    }
    return 0;
}