#pragma region include
#include <iostream>
#include <iomanip>
#include <stdio.h>

#include <sstream>
#include <algorithm>
#include <cmath>
#include <complex>

#include <string>
#include <cstring>
#include <vector>
#include <tuple>
#include <bitset>

#include <queue>
#include <complex>
#include <set>
#include <map>
#include <stack>
#include <list>

#include <fstream>
#include <random>
//#include <time.h>
#include <ctime>
#pragma endregion //#include
/////////
#define REP(i, x, n) for(int i = x; i < n; ++i)
#define rep(i,n) REP(i,0,n)
#define ALL(X) X.begin(), X.end()
/////////
#pragma region typedef
typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
typedef std::pair<LL,LL> PLL;//
typedef std::pair<int,int> PII;//
#pragma endregion //typedef
////定数
const int INF = (int)1e9;
const LL MOD = (LL)1e9+7;
const LL LINF = (LL)1e18+20;
const double PI = acos(-1.0);
const double EPS = 1e-9;
/////////
using namespace::std;

void solve(){
	string str;
	cin >> str;
	vector<int> num(26,0);
	for(int i=0;i<13;++i){
		num[ str[i]-'a' ]++;
	}
	bool flag = false;
	for(int i=0;i<13;++i){
		num[i]++;
		int one = 0;
		int two = 0;
		for(int j=0;j<13;++j){
			if( num[j] == 1 ){
				one++;
			}else if(num[j] == 2){
				two++;
			}
		}
		if( one == 12 && two == 1 ){
			cout << (char)('a'+i) << endl;
			flag = true;
		}
		num[i]--;
	}
	if( flag == false ){
		cout << "Impossible" << endl;
	}
}

#pragma region main
signed main(void){
	std::cin.tie(0);
	std::ios::sync_with_stdio(false);
	std::cout << std::fixed;//小数を10進数表示
	cout << setprecision(16);//小数点以下の桁数を指定//coutとcerrで別	

	solve();
}
#pragma endregion //main()