#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;
    unordered_map<char, int32_t> hist;
    for (auto c : s) {
        ++hist[c];
    }
    bool flag = false;
    for (auto i = 0; i < 13; ++i) {
        char c = char('a' + i);
        ++hist[c];
        int32_t n0 = 0, n1 = 0, n2 = 0;
        for (auto p : hist) {
            if (p.second == 1)
                ++n1;
            else if (p.second == 2)
                ++n2;
            else if (p.second != 0)
                ++n0;
        }
        if (n0 == 0 && n1 != 0 && n2 == 1) {
            cout << c << endl;
            flag = true;
        }
        --hist[c];
    }
    if (!flag) {
        cout << "Impossible" << endl;
    }
    return 0;
}