#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int main(){
	string S;
	cin >> S;
	char ans = '#';
	for (int i = 0; i < 26; i++){
		string T = S + (char) ('a' + i);
		vector<int> cnt(26, 0);
		for (int j = 0; j < 14; j++){
			cnt[T[j] - 'a']++;
		}
		sort(cnt.rbegin(), cnt.rend());
		if (cnt[6] == 2){
			ans = 'a' + i;
		}
	}
	if (ans == '#'){
		cout << "Impossible" << endl;
	} else {
		cout << ans << endl;
	}
}