#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    string s;
    cin >> s;
    int n = 13;
    int a[26] = {};
    rep(i,n)a[s[i] - 'a']++;

    int cnt2 = 0;
    int cnt1 = 0;
    char ans = 0;
    rep(i,26){
        if(a[i] == 1){
            cnt1++;
            ans = i+'a';
        }
        if(a[i] == 2)cnt2++;
    }

    if(cnt1 == 1 && cnt2 == 6){
        cout << ans << endl;
    }else{
        cout << "Impossible" << endl;
    }
    
    return 0;
}