#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define fi first
#define se second
template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<int, pii> pip;
const ll INF = 1e9+100;
const ll MOD = 1e9+7;
const double EPS  = 1e-10;
const bool debug = 0;
//---------------------------//

string s;
int cnt[26] = {};

int main() {
	cin >> s;
	REP(i, s.size()) cnt[s[i]-'a']++;
	
	char cc = '~';
	bool ng = false;
	REP(i, 26) {
		if (cnt[i] == 1) {
			if (cc == '~') cc = i;
			else ng = true;
		}
		else if (cnt[i] != 2 && cnt[i] != 0) ng = true;
	}
	
	if (ng) puts("Impossible");
	else printf("%c\n", cc + 'a');
	
	
	
	return 0;
}