#include<bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define reps(i,s,n) for(int i=(int)(s);i<(int)(n);i++)
const ll mod = 1e9 + 7;
const int INF = 1e9;

int main() {
	cin.sync_with_stdio(false);
	string s;
	cin >> s;
	sort(s.begin(), s.end());
	int c[26] = {};
	rep(i, s.size()) {
		if (s[i] == s[i + 1]) {
			c[s[i] - 'a']++;
			if (c[s[i] - 'a'] > 1) {
				cout << "Impossible" << endl;
				return 0;
			}
			s[i] = '0';
			s[i + 1] = '0';
			i++;
		}
	}
	int cnt = 0;
	char ans;
	rep(i, s.size()) {
		if (s[i] != '0') {
			cnt++;
			ans = s[i];
			if (c[ans - 'a'] != 0) {
				cout << "Impossible" << endl;
				return 0;
			}
		}
	}
	if (cnt != 1)cout << "Impossible" << endl;
	else cout << ans << endl;
	return 0;
}