# include <iostream>
# include <algorithm>
# include <vector>
# include <string>
# include <set>
# include <map>
# include <cmath>
# include <iomanip>
# include <functional>
# include <utility>
# include <stack>
# include <queue>
# include <list>
# include <bitset>
# include <complex>
# include <numeric>
# include <tuple>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
constexpr int MOD = 1000000000 + 7;
constexpr int INF = 2000000000;
constexpr int HINF = INF / 2;
constexpr double DINF = 100000000000000000.0;
constexpr long long LINF = 9223372036854775807;
constexpr long long HLINF = 4500000000000000000;
const double PI = acos(-1);
int dx[4] = { 0,1,0,-1 }, dy[4] = { 1,0,-1,0 };
#define ALL(x)   (x).begin(),(x).end()
#define mp       make_pair
#define eb       emplace_back

int C[30];
int main(){
	string s;
	cin >> s;
	for (int i = 0; i < s.size(); i++) {
		C[s[i] - 'a']++;
	}
	int a = 0;
	for (int i = 0; i < 30; i++) {
		if (C[i] == 1)a++;
	}
	if (a != 1) {
		cout << "Impossible" << endl;
		return 0;
	}
	for (int i = 0; i < 30; i++) {
		if (C[i] == 1) {
			cout << char(i + 'a') << endl;
			return 0;
		}
	}
}