#include <bits/stdc++.h>

#define VARNAME(x) #x
#define show(x) cerr << #x << " = " << x << endl

using namespace std;
using ll = long long;
using ld = long double;

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "sz:" << v.size() << "\n[";
    for (const auto& p : v) {
        os << p << ",";
    }
    os << "]\n";
    return os;
}

template <typename S, typename T>
ostream& operator<<(ostream& os, const pair<S, T>& p)
{
    os << "(" << p.first << "," << p.second
       << ")";
    return os;
}


constexpr ll MOD = (ll)1e9 + 7LL;
constexpr ld PI = static_cast<ld>(3.1415926535898);

template <typename T>
constexpr T INF = numeric_limits<T>::max() / 10;


int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    string s;
    cin >> s;
    vector<int> num(26, 0);
    for (const char c : s) {
        num[c - 'a']++;
    }
    int one = 0;
    int two = 0;
    int oneind = -1;
    for (int i = 0; i < 26; i++) {
        if (num[i] == 1) {
            one++;
            oneind = i;
        } else if (num[i] == 2) {
            two++;
        }
    }
    string ans;
    ans.push_back((char)('a' + oneind));
    cout << (one == 1 and two == 6 ? ans : "Impossible") << endl;


    return 0;
}