#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
typedef long long ll;
using namespace std;

int main() {
    string s;
    cin >> s;
    sort(s.begin(), s.end());
    s.erase(unique(s.begin(), s.end()), s.end());

    string t = "abcdefghijklm";
    if (s.size() == 12) {
        rep(i, s.size()) {
            if (s[i] != t[i]) {
                cout << t[i] << endl;
                break;
            }
        }
    } else if (s.size() == 13) {
        rep(i, t.size()) {
            cout << t[i] << endl;
        }
    } else {
        cout << "Impossible\n";
    }

    return 0;
}