#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
typedef long long ll;
using namespace std;

int main() {
    string s, t;
    cin >> s;
    t = s;

    int cnt[26] = {0};

    rep(i, s.size()) {
        cnt[s[i] - 'a']++;
    }

    bool flag = true;
    char ans;
    int one = 0;
    rep(i, 26) {
        if (cnt[i] == 1) {
            ans = 'a' + i;
            one++;
        }
        if (one >= 2 || cnt[i] >= 3) {
            flag = false;
            break;
        }
    }

    if (flag)
        cout << ans;
    else
        cout << "Impossible";
    cout << endl;

    return 0;
}