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

void solve() {
    std::vector<int> cnt(26, 0);

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

    for (int i = 0; i < 26; ++i) {
        ++cnt[i];

        bool ok = true;
        for (auto c : cnt) {
            if (c != 0 && c != 2) ok = false;
        }

        if (ok) {
            std::cout << char('a' + i) << std::endl;
            return;
        }

        --cnt[i];
    }

    std::cout << "Impossible" << std::endl;
}

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

    solve();

    return 0;
}