#include <bits/stdc++.h>
using namespace std;

int main() {
	string s;
	cin >> s;
	int a[26]{};
	for (auto c : s) {
		a[c - 'a']++;
	}
	int one = count(a, a + 26, 1), two = count(a, a + 26, 2);
	if (two == 6 && one == 1) {
		for (int i = 0; i < 26; i++) {
			if (a[i] == 1) {
				cout << char('a' + i) << endl;
				return 0;
			}
		}
	}
	cout << "Impossible" << endl;
	return 0;
}