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

int main(){
	cin.tie(0),ios::sync_with_stdio(false);
	string s; cin>>s;
	unordered_map<char,int>m;
	for(auto&&i:s)++m[i];
	string ans;
	for(auto&&i:m)if(i.second==1)ans+=i.first;
	cout<<(m.size()==7&&ans.size()==1?ans:"Impossible"s)<<"\n"s;
}