#include<iostream>
#include<string>
#include<cstring>
#include<complex>
#include<vector>
#include<algorithm>
#include<cstdlib>
#include<cmath>
#include<iomanip>
#include<numeric>
using namespace std;

#define REP(i, j) for (int i = 0; i < (int)j; ++i)
#define FOR(i, j, k) for (int i = (int)j; i < (int)k; ++i)
#define SORT(v) sort(v.begin(), v.end())
#define REVERSE(v) reverse(v.begin(), v.end())
typedef complex<double> P;
typedef vector<int> VI;
typedef long long LL;
typedef pair<int,int> PII;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	string s;
	cin >> s;

	string I = "Impossible";
	int alf[26] = {0};
	REP(i,s.length()) {
		alf[s[i] - 'a']++;
	}

	int cnt = 0, index;
	bool flg = false;
	REP(i,26) {
		if (alf[i] >= 3) flg = true;
		if (alf[i] == 1) {
			cnt++;
			index = i;
		}
	}

	char c = ('a' + index);
	if (flg && cnt != 1) cout << I << endl;
	else cout << c << endl;

	return 0;
}