#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<string>
#include<iostream>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include <sstream>
#include<iomanip>
#include <ctype.h>
using namespace std;

//#include<bits/stdc++.h>

int main() {

	string p;
	cin >> p;
	sort(p.begin(), p.end());
	vector<char>r;
	set<char>a;
	for (int i = 0; i < p.size(); i += 2) {
		a.insert(p[i]);//cout << a.size()<<p[i];
		if (p[i] != p[i + 1]){
			r.push_back(p[i]);
			i++;
			
			//cout << "Impossible" << endl; return 0;
	}

	}
	
	if(r.size()>1||a.size()!=6)cout << "Impossible" << endl;
	else cout << r[0] << endl;

	return 0;
}