#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]]++;
	}
	bool one  = false;
	char ans = '?';
	for(auto i : mp) {
		if(one && i.second == 1) {
			cout << "Impossible" << endl;
			return 0;
		}
		if(!one && i.second == 1) {
			one = true;
			ans = i.first;
		}
	}
	if(ans == '?') cout << "Impossible" << endl;
	else cout << ans << endl;
	return 0;
}