#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep3(i, l, n) for (int i = l; i < n; ++i)
#define chmax(a, b) a = (a >= b ? a : b)
#define chmin(a, b) a = (a <= b ? a : b)
#define out(a) cout << a << endl
#define outa(a, n) rep(_, n) cout << a[_] << " "; cout << endl
#define SZ(v) (int)v.size()
#define inf (int)(1e9+7)
#define abs(x) (x >= 0 ? x : -(x))

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    string s;
    cin >> s;
    int a[26] = {0};
    rep(i, 13) a[s[i] - 'a']++;
    // outa(a, 26);
    rep(i, 13) {
        if (a[i] != 1) break;
        if (i == 12) { rep(j, 13) out((char)('a' + j)); return 0; }
    }
    int c = 0, p = -1;
    rep(i, 13) {
        if (a[i] > 2) { out("Impossible"); return 0; }
        if (a[i] == 2) c++;
        if (a[i] == 0) p = i;
    }
    if (c >= 2) { out("Impossible"); return 0; }
    out((char)('a' + p));
}