#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,m,n) for(int i=m; i<int(n); ++i)
#define repll(i,m,n) for(ll i=m; i<ll(n); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define fi first
#define se second

template<typename T> bool chmin(T& a, T b){ if(a > b){a = b; return true;} return false; }
template<typename T> bool chmax(T& a, T b){ if(a < b){a = b; return true;} return false; }

template<typename T> T gcd(T a, T b){ return a % b ? gcd(b, a % b) : b; }
template<typename T> T lcm(T a, T b){ return a / gcd(a, b) * b; }

vector<pair<char, int>> run_length(string s){
    int n = s.size();
    vector<pair<char, int>> res;

    char pre = s[0];
    int cnt = 1;
    rep(i, 1, n){
        if(s[i] == pre){
            ++cnt;
        }else{
            res.push_back({pre, cnt});
            pre = s[i], cnt = 1;
        }
    }
    res.push_back({pre, cnt});

    return res;
}

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

    string s;
    cin >> s;

    sort(all(s));
    auto rle = run_length(s);

    if(rle.size() != 7){
        cout << "Impossible" << endl;
        return 0;
    }

    char ans = '?';
    for(auto [c, l] : rle){
        if(!(l == 1 || l == 2)){
            cout << "Impossible" << endl;
            return 0;
        }
        if(l == 1){
            if(ans != '?'){
                cout << "Impossible" << endl;
                return 0;
            }
            ans = c;
        }
    }
    cout << ans << endl;

    return 0;
}