#include <iostream>
#include <stdio.h>
#include <cstring>
#include <math.h>
#include <algorithm>
#include <vector>
#include <string>
#include <stdlib.h>
#include <queue>
#include <stack>
#include <utility>
#define rep(i,n) for(int i=0;i<n;i++)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define FOR(i,a,b) for(int i=a;i<b;i++)
#define ll long long
#define INF 1000000001
#define mod 1000000007
using namespace std;

int main(){
	//cin.tie(0);
	//ios::sync_with_stdio(false);
	
	char s[14];
	cin>>s;
	int a[26]={0};
	rep(i,13){
		a[int(s[i])-97]++;
	}
	int ans=-1;
	rep(i,26){
		if(a[i]>2){
			cout<<"Impossible"<<endl;
			return 0;
		}else if(a[i]==1){
			if(ans!=-1){
				cout<<"Impossible"<<endl;
				return 0;
			}else{
				ans=i;
			}
		}
	}
	cout<<char(ans+97)<<endl;
}