#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using ul = unsigned long;
using ull = unsigned long long;

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

	string s;
	cin >> s;

	map<char, int> m;
	for (const auto& it : s)
		++m[it];

	bool ok = true;
	int odd{ 0 };
	char c;
	for (const auto& it : m)
	{
		if (it.second > 2)
		{
			ok = false;
			break;
		}
		else if (it.second == 1)
		{
			c = it.first;
			++odd;
		}
	}

	if (!ok || odd > 1)
		cout << "Impossible\n";
	else
		cout << c << "\n";

	return 0;
}