#include <iostream>
#include <string>
#include <math.h>
#include <algorithm>
using namespace std;
typedef long long int ll;
typedef pair<ll, ll> P;
int main() {
	string s;
	cin >> s;
	ll cnt[27] = {}, a, A = 0, B = 0;
	char ans;
	for (int i = 0; i < 13; i++) {
		a = s[i] - 'a' + 1;
		cnt[a]++;
	}
	for (int i = 1; i <= 26; i++) {
		if (cnt[i] == 2)A++;
		if (cnt[i] == 1) {
			B++;
			ans = 'a' + i - 1;
		}
	}
	if (A == 6 && B == 1)cout << ans << endl;
	else cout << "Impossible" << endl;
	return 0;
}