#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <vector>
#include <cmath>
#include <string>
#include <sstream>
#include<sstream>
#include <queue>
#include <limits>
#include <cstdlib>
#include <deque>
#include <map>
#include <set>
#include <iomanip>
using namespace std;



int main() {

	string s;
	cin>>s;

	int alph[26] ={};

	for(int i=0; i<s.length(); i++){
		alph[(int)s[i]-(int)'a'] += 1;
	}

	int alphcount=0;
	int two_times=0;
	int ans=0;
	for(int i=0; i<26; i++){

		if(alph[i] == 2){
			two_times++;
		}
		if(alph[i] !=0){
			alphcount++;
		}
		if(alph[i] ==1){
			ans = i;
		}
	}

	if(alphcount ==7 && two_times ==6){
		cout<<(char)(ans+(int)'a')<<endl;
	}else {
		cout<<"Impossible"<<endl;
	}
	return 0;
}