#include<bits/stdc++.h>
using namespace std;
using ll = long long;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("Yes")
#define p_no() p("No")

const ll mod = 1e9 + 7;
const ll inf = 1e18;

void impossible(){
    p("Impossible");
    exit(0);
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    string s; cin >> s;
    map<char, ll> mp;

    for(char c : s){
        mp[c]++;
    }

    vector<ll> A;
    for(auto pa : mp){
        A.push_back(pa.second);
    }
    sort(ALL(A));

    if(A.size()==7 && A[0]==1 && A[1]==2 && A.back()==2){
        for(auto pa : mp){
            if(pa.second==1){
                p(pa.first);
                return 0;
            }
        }
    }else{
        impossible();
    }
    
    return 0;
}