#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<string>
#include<stack>
#include<queue>
#include<vector>
#include<algorithm>
#include<iomanip>
#include<cassert>
typedef long long int ll;
using namespace std;
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define REP(i,n) for (int i=0;i<(n);i++)
#define EREP(i,n) for (int i=1;i<=(n);i++)
const int MOD = 1000000007;
const int INF = 1000000;

    string S;
     int Y=0,E=0,A[15],H=0,EX=0;
    bool fir=true;
int main(){
    ios::sync_with_stdio(false);
    cin>>S;
    sort(S.begin(),S.end());
    string as="Impossible";
    REP(i,13){
        A[((int)S[i]-'a')]++;

    }
    REP(i,26){
        if(A[i]==0)continue;
        if(A[i]==1){
            if(!fir){as="Impossible";break;}
            as=(char)(i+'a');fir=false;
        }
    }
    cout<<as<<endl;
}