#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for(int i = 0; i < n; i++)

int main() {
    string s;
    cin >> s;

    vector<int> cnt(26);
    REP(i, s.size()) cnt[s[i] - 'a']++;

    int one = 0, two = 0;
    char ans;
    REP(i, 26) {
        switch (cnt[i]) {
            case 1:
                one++;
                ans = (char) i+'a';
                break;
            case 2:
                two++;
                break;
        }
    }

    if (one == 1 && two == 6) {
        cout << ans << endl;
    } else {
        cout << "Impossible" << endl;
    }
    return 0;
}