#include <iostream>
#include <string>
#include <map>
using namespace std;
int main(int argc, char* argv[])
{
	string s;
	cin>>s;
	int i;
	map<char,int> aMap;
	map<char,int>::iterator mit;
	for (i=0;i<s.length();i++){
		mit=aMap.find(s[i]);
		if (mit==aMap.end()){
			aMap[s[i]]=1;
		}else{
			(*mit).second++;
		}
	}
	char ch;
	char n=0;
	for (mit=aMap.begin();mit!=aMap.end();mit++){
		int num=(*mit).second;
		if (num>2){
			cout<<"Impossible"<<endl;
			return 0;
		}else if (num==1){
			ch=(*mit).first;
			n++;
		}
	}
	if (n==1){
		cout<<ch<<endl;
	}else{
		cout<<"Impossible"<<endl;
	}
	return 0;
}