#include<bits/stdc++.h>
#define rep(i,a,n) for (int i = a;i < n;i++)
using namespace std;

int main() {
    string s; cin >> s;
    map<char, int> mp;
    rep(i, 0, s.size()) {
        mp[s[i]]++;
    }
    int one = 0, two = 0;
    string ans = "Impossible";
    for (auto i : mp) {
        one += i.second == 1;
        two += i.second == 2;
        if (i.second == 1) ans = i.first;
    }
    cout << (one == 1 && two == 6 ? ans : "Impossible") << endl;
}