#include<iostream>
	#include<vector>
	#include<algorithm>
	#include <string>
	#include<cmath>
	#include<map>
	#include<cstdio>
	#include<stack>
	#include<queue>

	using namespace std;

	#define ok printf("ok\n");
	#define ok1 printf("ok1\n");
	#define ok2 printf("ok2\n");
	#define MAX 1000000
	#define rep(i,n) for(int i=0;i<n;i++)
	#define REP(i,s,n) for(int i=(s);i<(n);i++)
	#define repr(i,n) for(int i=n-1;i>=0;i--)
	#define REPR(i,s,n) for(int i=(s);i>=(g);(i)--)
	#define all(a) (a).begin(),(a).end()
	#define reall(a) (a).rbegin(),(a).rend()
	#define pb push_back
	#define s string

	typedef vector<int> vi;
	typedef vector<s> vs;
	typedef long long ll;
	typedef vector<ll> vll;
	int facctorialMethod(int k);
	int gcd(int a, int b);
	int main()
	{
		s s,t;
		int a = 0;
		cin >> s;
		sort(all(s));
		//cout << s;
		for(int i=0;i < s.size();i=i+2)
		{
			//cout <<i;
			if(s[i] == s[i+1])
				continue;
		else if(a == 0)
			{
				a++;
				t = s[i];
				i += 1;
				//cout << s[i-1] << s[i] << s[i+1];
				if(s[i-1] == s[i-3])
				{
					cout << "Impossible";
					return 0;
				}
			}
			else
				{
					cout << "Impossible";
					return 0;
				}
		}
		cout << t;
		return 0;
	}