#include "bits/stdc++.h"
using namespace std;
#define fastcin {\
    cin.tie(0);\
    ios::sync_with_stdio(false);\
}
#define rep(i, a, b) for(int i = a; i < b; i++)
#define rrep(i, a, b) for(int i = a; i > b; i--)
#define fore(i, a) for(auto &i:a)
#define print(x) cout << x << "\n"
#define SORT(a, n) sort(a, a+n);
#define REVERSE(a,n) reverse(a,a+n);
#define VSORT(v) sort(v.begin(), v.end());
#define VREVERSE(v) reverse(v.begin(), v.end());
#define MOD 1000000007
typedef long long ll;

int main() {
    fastcin;
    string S;
    cin >> S;
    map<char, int> m;
    rep(i, 0, 13) m[S[i]]++;
    int b;
    char ans;
    fore(p, m) {
        if(p.second==1) {
            b++;
            if(b > 1) {
                cout << "Impossible" << endl;
                return 0;
            } else ans = p.first;
        }
    }
    cout << ans << endl;
    return 0;
}