#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream> 
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include<fstream>
using namespace std;
#define eps 0.000000001
#define LONG_INF 10000000000000000
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define f(n) for(int i=0;i<n;++i)
#define REP(i,n) for(long long i = 0;i < n;++i)
int main() {
	string s;
	cin >> s;
	sort(s.begin(), s.end());
	map<int, int> wow;
	for (int i = 0;i < s.length();++i) {
		wow[s[i]]++;
	}
	int geko = 0;
	int cnt = 0;
	for (int i = 0;i < 13;++i) {
		cnt += wow['a'+i];
		geko = max(geko, wow['a' + i]);
	}
	if (cnt == 13) {
		if (geko == 1) {
			for (int i = 0;i < 13;++i) {
				if (wow['a'+i] != 0) {
					cout << (char)('a' + i) << endl;
				}
			}
			return 0;
		}else if(geko == 2){
			for (int i = 0;i < 13;++i) {
				if (wow['a' + i] == 0) {
					cout << (char)('a' + i) << endl;
					return 0;
				}
			}
		}
	}
	cout << "Impossible" << endl;
	return 0;
}