#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 <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
using namespace std;
#define Ma_PI 3.141592653589793
#define eps 0.00000000000000000000000001
#define LONG_INF 3000000000000000000
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define REP(i,n) for(long long i = 0;i < n;++i)                                                                             
#define seg_size 524288
int main() {
	string s;
	cin >> s;
	map<int, int> wow;
	for (int i = 0;i < s.length();++i) {
		wow[s[i]]++;
	}
	int tmp = 0;
	for (auto i = wow.begin();i != wow.end();++i) {
		if (i->second == 1) {
			if (tmp != 0) {
				cout << "Impossible" << endl;
				return 0;
			}
			tmp = i->first;
		}else if(i -> second != 2){
			cout << "Impossible" << endl;
			return 0;
		}
	}
	cout << (char)tmp << endl;
	return 0;
}