#include <bits/stdc++.h>

using namespace std;

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

    map<char, int> mp;
    for (char i : s) {
        mp[i]++;
    }

    int one = 0, two = 0;
    string ans;
    for (char c = 'a'; c <= 'z'; ++c) {
        switch (mp[c]) {
            case 1:
                one++;
                ans = c;
                break;
            case 2:
                two++;
                break;
        }
    }

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