#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {

	string s;
	cin >> s;

	int num[13] = { };

	string ans ="abcdefghijklm";
	int count = 0;

	for (int i = 0; i < s.length(); i++) {
		if (s[i] - 'a' > 12) {
			cout << "Impossible" << endl;
			return 0;
		}

		num[s[i] - 'a']++;

		if (num[s[i] - 'a'] == 2) {
			count++;
			if (count == 2) {
				cout << "Impossible" << endl;
				return 0;
			}
		}
		/*cout<<num[s[i] - 'a']<<endl;*/
		if (num[s[i] - 'a'] > 2) {
			cout << "Impossible" << endl;
			return 0;
		}
	}

	if(count==1){
		for(int i=0; i<13; i++){
			if(num[i] ==0){
				cout<<ans[i]<<endl;
			}
		}
	}else {
		for(int i=0; i<13; i++){
			if(num[i] ==1){
				cout<<ans[i]<<endl;
			}
		}
	}

	return 0;
}