#include <iostream>
#include <vector>
#include <string>

void solve() {
    std::vector<int> cnt(13, 0);

    {
        std::string s;
        std::cin >> s;
        for (char c : s) {
            ++cnt[c - 'a'];
        }
    }

    bool pos = false;
    for (int c = 0; c < 13; ++c) {
        ++cnt[c];

        int two = 0;
        for (auto x : cnt) {
            if (x > 2) two = 2;
            if (x == 2) ++two;
        }

        if (two == 1) {
            std::cout << char('a' + c) << std::endl;
            pos = true;
        }

        --cnt[c];
    }

    if (!pos) std::cout << "Impossible" << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}