#include <algorithm>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <stack>
#include <vector>


using namespace std;


#define ll long long
#define ld long double

#define all(v) begin(v), end(v)
#define rep(i, n)     for (ll i = 0; i < n;  ++i)
#define rep2(i, n, m) for (ll i = n; i <= m; ++i)
#define rep3(i, n, m) for (ll i = n; i >= m; --i)

template<class S, class T> bool chmax(S& a, T& b) { if (a < b) { a = b; return true; } return false; }
template<class S, class T> bool chmin(S& a, T& b) { if (a > b) { a = b; return true; } return false; }


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

    map<char,int> cnt;
    for (char& c : s) ++cnt[c];

    string ans;

    for (auto [c, p] : cnt) {
        if (p > 2) {
            cout << "Impossible" << endl;
            return (0);
        } if (p == 1) ans += c;
    }

    if (ans.size() == 1) cout << ans << endl;
    else cout << "Impossible" << endl;

    return (0);
}