#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#define p(s) cout<<(s)<<endl
#define REP(i,n,N) for(int i=n;i<N;i++)
#define RREP(i,n,N) for(int i=N-1;i>=n;i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define F first
#define S second
typedef long long ll;
using namespace std;
const int mod = 1e9+7;

string s;
int C[30];
int main(){
    cin>>s;
    for(auto c:s) C[c-'a']++;
    string ans="Impossible";
    REP(i,0,'z'-'a'+1){
        if(C[i]==1) ans = (char) ('a' + i);
        else if(C[i]==2||C[i]==0) continue;
        else{
            ans="Impossible";
            break;
        }
    }
    p(ans);
    return 0;
}