#include <bits/stdc++.h>

using namespace std;

string S;
int cnt[30];

int main() {
    cin >> S;

    bool f = false;

    for (char c = 'a'; c <= 'm'; c++) {
        string s = S + c;
        sort(s.begin(), s.end());
        memset(cnt, 0, sizeof(cnt));
        for (char cc : s) cnt[cc - 'a']++;
        bool ok = false;
        for (char cc = 'a'; cc <= 'm'; cc++) {
            if (cnt[cc - 'a'] == 0) {
                ok = false;
                break;
            } else if (cnt[cc - 'a'] == 2) {
                if (ok) {
                    ok = false;
                    break;
                } else {
                    ok = true;
                }
            }
        }
        if (ok) {
            cout << c << endl;
            f = true;
        }
    }

    if (!f) cout << "Impossible" << endl;
}