#include <bits/stdc++.h>
using namespace std;

int main()
{
	string S;
	cin >> S;

	map<char, int> mp;
	for( int i = 0; i < S.size(); i++ ) {
		mp[ S[i] ]++;
	}
	int cnt = 0;
	for( auto e : mp ) {
		if( e.second == 2 ) cnt++;
	}

	if( cnt != 6 ) cout << "Impossible" << endl;
	else {
		for( auto e : mp ) {
			if( e.second == 2 ) continue;
			cout << e.first << endl;
			break;
		}
	}
}