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

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0); 

	string s;
	cin >> s;
	map<char, int> mp;
	for(int i = 0; i < s.size(); i++) {
		mp[s[i]]++;
	}
	int one = 0, two = 0;
	char ans;
	for(auto i : mp) {
		if(i.second == 2) two++;
		if(i.second == 1) {
			one++;
			ans = i.first;
		}
	}
	if(one == 1 && two == 6) {
		cout << ans << endl;
	} else {
		cout << "Impossible" << endl;
	}
	return 0;
}