#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>
#include <string>
#include <map>

using namespace std;

int main() {
	string s;
	cin >> s;

	map<char, int> alpha;
	for (char c : s) {
		alpha[c]++;
	}

	string ans = "Impossible";
	for (auto x : alpha) {
		if (x.second == 1) {
			if (ans.size() == 1) {
				ans = "Impossible";
				break;
			} else {
				ans = x.first;
			}
		} else if (x.second > 2) {
			ans = "Impossible";
			break;
		}
	}

	cout << ans << endl;
	return 0;
}