#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;



int main() {
	int i, j, k;
	string s;
	map<char, int>mp;



	cin >> s;

	for (i = 0; i < s.length(); i++) {
		if (mp.count(s[i]) == 0) {
			mp.insert(make_pair(s[i], 1));
		}
		else {
			mp[s[i]]++;
		}
	}

	int one = 0, two = 0;
	char ch;

	if (mp.size() != 7) {
		cout << "Impossible" << endl;
		return 0;
	}
	else if (mp.size() == 7) {
		for (auto itr = mp.begin(); itr != mp.end(); itr++) {
			(*itr).second == 2 ? two++ : (*itr).second == 1 ? one++,ch=(*itr).first : 0;
		}
	}

	(two == 6 && one == 1 ) ? cout << ch << endl : cout << "Impossible" << endl;






	getchar();
	getchar();
	return 0;
}