#include <iostream>
#include <map>
#include <string>

int main() {
    using namespace std;
    string s;
    cin >> s;
    map<char, int> cc;
    for (const char c: s) {
        if (cc.find(c) == cc.end()) {
            cc[c] = 1;
        } else {
            cc[c]++;
        }
    }
    if (cc.size() != 7) {
        cout << "Impossible" <<endl;
        return 0;
    } else {
        char res;
        for (const pair<char, int> n: cc) {
            if (n.second == 1) {
                res = n.first;
            } else if (n.second > 2) {
                cout << "Impossible" <<endl;
                return 0;
            }
        }
        cout << res << endl;
    }
}