#include <algorithm>
#include <array>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <optional>
#include <queue>
#include <sstream>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

int main() {
    string s;
    cin >> s;
    vector<int32_t> hist(26);
    for (auto c : s) {
        ++hist[c - 'a'];
    }
    vector<int32_t> ns(2);
    for (auto i = 0; i < 26; ++i) {
        if (hist[i] == 1 || hist[i] == 2)
            ++ns[hist[i] - 1];
    }
    int32_t ans = -1;
    if (ns[0] == 1 && ns[1] == 6) {
        for (auto i = 0; i < 26; ++i) {
            if (hist[i] == 1)
                ans = i;
        }
    }
    if (ans != -1) {
        cout << char('a' + ans) << endl;
    } else {
        cout << "Impossible" << endl;
    }
    return 0;
}