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

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

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

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

#include <ctime>
////
//#include <random>//
#pragma endregion //#include
/////////

#pragma region typedef
typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
#pragma endregion //typedef
////定数
const int INF = (int)1e9;
const LL MOD = (LL)1e9+7;
const LL LINF = (LL)4e18+20;
const LD PI = acos(-1.0);
const double EPS = 1e-9;
/////////
using namespace::std;

void solve(){
	vector<int> hist(26,0);
	string str;
	cin>> str;
	int size = str.size();
	for(int i=0;i<size;++i){
		int ter = str[i]-'a';
		hist[ter]++;
	}
	int ans = -1;
	for(int i=0;i<26;++i){
		if( hist[i] > 2 ){
			cout << "Impossible" << endl;
			return;
		}
		if( hist[i]==2 ){
		}else if(hist[i]==1){
			if( ans==-1 ){
				ans = i;
			}else{
				cout << "Impossible" << endl;
				return;
			}
		}
	}
	string ansStr = " ";
	ansStr[0] = 'a'+ans;
	cout << ansStr << 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()