#include <iostream>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <map>
#include <queue>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using namespace std;

template<class T>
constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    string s;
    cin >> s;
    map<char,int> m;
    for (auto &&i : s) m[i]++;
    int p = 0;
    char c;
    for (auto &&i : m) {
        if(i.second >= 3 || p > 1){
            cout << "Impossible\n";
            return 0;
        }else if(i.second == 1){
            p++;
            c = i.first;
        }
    }
    cout << c << "\n";
    return 0;
}