#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;

int A[26];

int main()
{
	string S; cin >> S;
	for (int i = 0; i < S.size(); i++) {
		A[S[i] - 'a']++;
	}

	int cnt = 0;
	int ans = -1;
	for (int i = 0; i < 26; i++) {
		if (A[i] == 1) {
			cnt++;
			ans = i;
		}
		else if (A[i] > 2) {
			ans = -1;
			cnt = -100;
		}
	}

	if (cnt == 1)cout << char(ans + 'a') << endl;
	else cout << "Impossible" << endl;

    return 0;
}