#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#define REP(i, m, n) for(int i=int(m);i<int(n);i++)
#define EACH(i, c) for (auto &(i): c)
#define all(c) begin(c),end(c)
#define EXIST(s, e) ((s).find(e)!=(s).end())
#define SORT(c) sort(begin(c),end(c))
#define pb emplace_back
#define MP make_pair
#define SZ(a) int((a).size())

//#define LOCAL 0
//#ifdef LOCAL
//#define DEBUG(s) cout << (s) << endl
//#define dump(x)  cerr << #x << " = " << (x) << endl
//#define BR cout << endl;
//#else
//#define DEBUG(s) do{}while(0)
//#define dump(x) do{}while(0)
//#define BR
//#endif


//改造
typedef long long int ll;
using namespace std;
#define INF (1 << 20)
#define INFl (ll)5e15
#define DEBUG 0 //デバッグする時1にしてね

//ここから編集する


int main() {
    string s;
    cin >> s;
    map<char,int> mp;
    REP(i,0,s.size()){
        mp[s[i]]++;
    }


    char addch = '0';
    for(auto it = mp.begin(); it != mp.end(); it++){
        if(it->second == 2) continue;
        if(it->second == 1) {
            if (addch == '0')
                addch = it->first;
            else {
                cout << "Impossible" << endl;
                return 0;
            }
        }
        if(it->second > 2){
            cout << "Impossible" << endl;
            return 0;
        }
    }
    cout << addch << endl;

    return 0;
}