#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	string S;
	cin >> S;
	vector<int> A(26, 0);
	char res;
	for (int i = 0; i < 13; i++) {
		A[(int)(S[i] - 'a')]++;
	}
	int c = 0;
	bool f = true;
	for (int i = 0; i < 26; i++) {
		if (A[i] == 1) {
			c++;
			res = i + 'a';
		}
		else if (A[i] > 2) {
			f = false;
		}
	}
	if (c != 1) {
		f = false;
	}
	if (f) {
		cout << res << endl;
	}
	else {
		cout << "Impossible" << endl;
	}
}