#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
#define REP(i,n) for(int i=0; i<int(n); i++)
#define FOR(i,m,n) for(int i=int(m); i<int(n); i++)
#define ALL(obj) (obj).begin(),(obj).end()
#define VI vector<int>
#define VLL vector<long long>
#define VVI vector<vector<int>>
#define VVLL vector<vector<long long>>
#define VC vector<char>
#define VS vector<string>
#define VVC vector<vector<char>>
#define fore(i,a) for(auto &i:a)

typedef pair <int, int> P;
template<class T> bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }
const int  INF = 2147483647;
const ll INFL = 9223372036854775807;
const ll mod = 1000000007;





int main() {
	
	string s;
	cin >> s;

	VI v(30, 0);

	REP(i, 13) {
		int n = s[i] - 'a';
		v[n]++;
	}

	char c = '.';

	REP(i, 30) {
		if (v[i] >= 3) {
			cout << "Impossible" << endl;
			return 0;
		}
		if (v[i] == 1) {
			if (c == '.')c = 'a' + i;
			else {
				cout << "Impossible" << endl;
				return 0;
			}
		}
	}

	cout << c << endl;

	return 0;
}