#define _USE_MATH_DEFINES
#include<iostream>
#include<vector>
#include<algorithm>
#include<cmath>
#include<string>
#include<iomanip>
#include<numeric>
#include<queue>
#include<deque>
#include<stack>
#include<set>
#include<map>
#include<random>
using namespace std;
typedef long long ll;
const int mod=1e9+7;
int main(){
	string s;
	cin>>s;
	int arr[26]={};
	for(auto x:s)
		arr[x-'a']++;
	int cnt=0;
	for(int i=0;i<26;i++)
		if(arr[i]!=2&&arr[i]!=0)
			cnt++;
	if(cnt!=1)
		cout<<"Impossible"<<endl,exit(0);
	else
		for(int i=0;i<26;i++)
			if(arr[i]==1)
				cout<<char(i+'a')<<endl,exit(0);
	cout<<"Impossible"<<endl;
}