#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

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

    vector<pair<int, char> > v(26);
    for(int i=0; i<26; ++i)
        v[i].second = 'a' + i;
    for(int i=0; i<13; ++i)
        ++ v[s[i]-'a'].first;

    sort(v.rbegin(), v.rend());
    if(v[5].first == 2 && v[6].first == 1)
        cout << v[6].second << endl;
    else
        cout << "Impossible" << endl;

    return 0;
}