#include <iostream>
#include <vector>
#include <algorithm>
#include <math.h>
#include <map>
#include <set>

#define ALL(x) (x).begin(),(x).end()
#define RALL(x) (x).rbegin(),(x).rend()
#define REP(i,a,n) for(int i = (a);i < (n);i++)
#define PI 3.14159265359
#define MOD 1000000007

using namespace std;

int main(){
    vector<int> a(26);
    string s,al = "abcdefghijklmnopqrstuvwxyz",ans;
    cin >> s;
    REP(i, 0, s.length()){
        int index = s[i]-'a';
        a[index]++;
    }
    REP(i, 0, 26){
        if(a[i] == 1)ans += al[i];
        if(a[i] > 2)ans = "Impossible";
    }
    if(ans.length() != 1)ans = "Impossible";
    cout << ans << endl;
    return 0;
}