#include <iostream>
#include <vector>
#include <list>
#include <algorithm>
#include <string>
#include <sstream>
#include <stack>
#include <iomanip>
#include <numeric>
#include <queue>
#include <climits>
#include <set>
#include <complex>
#include <cmath>
#include <cstring>
#include <map>
using namespace std;
using ll = long long;
#define MOD 1000000007
#define INF 1LL << 59
using ld = long double;

bool operator<(const pair<int, int> &a, const pair<int, int> &b)
{
    if (a.first == b.first)
    {
        return b.second > a.second;
    }
    return a.first < b.first;
}

int a[26];
int main()
{
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    //cout << fixed << setprecision(6);
    string s;
    cin >> s;
    int n = s.size();
    for (int i = 0; i < n; ++i)
    {
        a[s[i] - 'a']++;
    }

    bool ce = true;
    int cntt = 0;
    int cnts = 0;
    char ans;
    for (int i = 0; i < 26; ++i)
    {
        if (a[i] == 2)
            cntt++;
        if (a[i] == 1)
        {
            cnts++;
            ans = (char)('a' + i);
        }
    }

    if (cntt == 6 && cnts == 1)
    {
        cout << ans << endl;
    }
    else
    {
        cout << "Impossible" << endl;
    }

    return 0;
}