#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>
#include <iterator>

typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;

ll gcd(ll a, ll b) {
    if (a < b)swap(a, b);
    if (b == 0)return a;
    return gcd(b, a % b);
}
int lcm(int a, int b) {
    return a * b / gcd(a, b);
}

long long mod_exp(long long b, long long e, long long m) {
    if (e > 0 && b == 0) return 0;
    long long ans = 1;
    b %= m;

    while (e > 0) {
        if ((e & 1LL)) ans = (ans * b) % m;
        e = e >> 1LL;
        b = (b * b) % m;
    }

    return ans;
}


int main()
{

    int Q;
    Q = 1;
    //cin >> Q;
    
    while (Q--) {
        
        string p;
        cin >> p;
        int x = 0;
        sort(p.begin(), p.end());
        set<char>s;
        map<char, int>mp;
        for (int i = 0; i < p.size(); i++) {
            mp[p[i]]++;
            s.insert(p[i]);
        }
        for (int i = 0; i < p.size(); i++) {
            if (mp[p[i]] > 2) {
                cout << "Impossible"; return 0;
            }
            if (mp[p[i]] == 2)x++;
        }
        
        if (x == 12 && s.size() == 7) {
            for (int i = 0; i < p.size(); i+=2) {
                if (p[i] == p[i + 1])continue;
                cout << p[i]; return 0;
            }
            cout << p[p.size() - 1];
        }
        else cout<<"Impossible";
        
    }
    return 0;
}