#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <map>
#include <stack>
#include <cmath>
#include <queue>
#include <numeric>

int main()
{
	std::string s;

	std::cin >> s;

	std::map<char, int> m;

	for (size_t i = 0; i < s.length(); i++)
	{
		m[s[i]]++;
	}

	int count = 0;
	char c;

	for (auto i = m.begin(); i != m.end(); i++)
	{
		if (i->second > 2) {
			std::cout << "Impossible" << std::endl;
			return 0;
		}
		if (i->second == 1) {
			count++;
			c = i->first;
		}
	}

	if (count == 0 || count > 1) {
		std::cout << "Impossible" << std::endl;
	}
	else {
		std::cout << c << std::endl;
	}

	return 0;
}