#include <iostream>
#include <map>
using namespace std;
int main(void) {
	string s;
	cin >> s;
	map<char, int> mp;
	for (int i = 0; i < 13; ++i) {
		++mp[s[i]];
	}
	bool ok = true;
	char ans = ' ';
	if (mp.size() != 7) ok = false;
	else {
		for (auto x : mp) {
			if (x.second != 1 && x.second != 2) {
				ok = false;
				break;
			}
			if (x.second == 1)
				ans = x.first;
		}
	}
	if (ok) cout << ans << endl;
	else cout << "Impossible" << endl;
	return 0;
}