#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

//#define int long long
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define all(c) begin(c),end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = (int)(1e9) + 7;
template<class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	string s;
	cin >> s;
	bool flag = true;
	for (char c = 'a'; c < 'z' + 1; c++) {
		string t = s;
		t += c;
		bool x, y; x = 0, y = 0;
		int a[128] = {};
		rep(i, 0, t.size()) {
			a[t[i]]++;
		}
		for (char d = 'a'; d < 'm' + 1; d++) {
			if (a[d] == 0) {
				x = false;
				break;
			}
			if (x&&a[d] == 2) {
				x = false;
				break;
			}
			if (a[d] == 2) {
				x = true;
			}
		}
		if (x) {
			cout << c << endl;
			flag = false;
		}
	}
	if (flag)cout << "Impossible" << endl;
	return 0;
}