#include <bits/stdc++.h>
#include <typeinfo>
#include <cxxabi.h>
#ifdef LOCAL
#include "dbgtoki.hpp"
#define DUMP(i) dump((string)TOSTRING(i), demangle(typeid(i).name()), __LINE__ , i) 
#else
#define DUMP(i)
#endif

using namespace std;
#define TOSTRING(x) #x
#define SZ(x) (int)(x).size()
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define ALL(s) (s).begin(), (s).end()
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()) , v.end());
#define dis distance
#define so sort
#define rev reverse
#define pub push_back
#define sec second
#define fir first

typedef long long unsigned int llu;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vector <int> > vvi;
const int EPS = 1e-9;
const int MOD = 1e9+7;
const int INF = (1 << 30);

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	string s;
	cin >> s;
	map<char,int> M;
	REP(i,13){
		M[s[i]]++;
	}

	string ans = "";
	for(auto m:M){
		switch(m.sec){
			case 1:
				if(ans == ""){
					ans = m.fir;
				}else{
					cout << "Impossible" << endl;
					return 0;
				}
				break;
			case 2:
				break;
			case 0://ないとは思うが一応
				break;
			default:
				cout << "Impossible" << endl;
				return 0;
		}
	}
	cout << ans << endl;
	return 0;
}