#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";
    cin >> s;
    REP(i, 0, s.length()){
        int index = s[i]-'a';
        a[index]++;
    }
    if(count(ALL(a), 1) != 1)cout << "Impossible\n";
    else{
        REP(i, 0, 26){
            if(a[i] == 1)cout << al[i] << endl;
        }
    }
    return 0;
}