#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    string s; cin >> s;
    int ok = 0;
    for(char c = 'a'; c <= 'm'; c++) {
        s.push_back(c);

        map<char,int> mp;
        for(char x : s) mp[x]++;
        int c2 = 0;
        for(auto e : mp) if(e.second == 2) c2++;
        if(c2 == 1 && mp.size() == 13u) {
            cout << s.back() << endl;
            ok = 1;
        }
        s.pop_back();
    }
    if(!ok) {
        cout << "Impossible" << endl;
    }
}