#include <bits/stdc++.h>
#define all(r) r.begin(),r.end()
using namespace std;
struct Solution {
    void solve(std::istream& in, std::ostream& out) {
        string s, t;
        in >> s;
        sort(all(s));
        t = s;
        t.erase(unique(all(t)), t.end());
        if (t.size() == 7) {
            string ans = "";
            for (int i = 0; i < 13; i++) {
                if (s[i] != t[i / 2]) {
                    ans =s[i];
                    break;
                }
            }
            out << ans << '\n';
        }
        else {
            out << "Impossible" << '\n';
        }
    }
};
void solve(std::istream& in, std::ostream& out) {
    out << std::setprecision(12);
    Solution solution;
    solution.solve(in, out);
}
#include <fstream>
#include <iostream>
int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    istream& in = cin;
    ostream& out = cout;
    solve(in, out);
    return 0;
}