#include<iostream>
#include<string>
#include<algorithm>
#include<functional>
#include<vector>
#include<set>
#include<queue>
#include<list>
#include<stdio.h>
#include<stdlib.h>
#include<map>

using namespace std;

int main() {
	string s;
	cin >> s;
	
	map<char, int> data;

	for (int i = 0; i < s.length(); ++i) {
		++data[s[i]];
	}

	map<char, int>::iterator itr;

	char one = -1;
	int two = 0;
	for (auto itr = data.begin(); itr != data.end(); ++itr) {
		if (itr->second==1) {
			one = itr->first;
		}
		else if (itr->second==2) {
			++two;
		}
	}

	if (two == 6 && one != -1) {
		cout << one;
	}
	else {
		cout << "Impossible";
	}
}