#include <iostream>
#include <math.h>
#include <algorithm>
#include <functional>
#include <vector>
#include <typeinfo>
#include <climits>
#include <ctype.h>
#include <string>
#include <stack>
using namespace std;
#define FOR(i, a, n) for (int i = a; i < n; i++)
#define REP(i, n) for(int i = 0; i < n; i++)
struct omt{omt(){ios::sync_with_stdio(false);cin.tie(0);}}star;

int main(){
    string s;
    cin >> s;

    int A[26] = {};
    REP(i, s.size()){
        A[s[i] - 97]++;
    }
    int o = 0, t = 0, idx;
    REP(i, 26){
        if(A[i] == 1){
            o++;
            if(o > 1) break;
            idx = i;
        }
        else if(A[i] == 2) t++;
    }
    if(o == 1 and t == 6){
        char c = 'a' + idx;
        cout << c << endl;
    }else cout << "Impossible" << endl;


    return 0;
}