#include <iostream>
using namespace std;

int main() {
	char s[13];
	char c1 = 'N';
	char c2 = 'N';
	int OK = 1;
	for (int i = 0; i != 13; i++) {
		cin >> s[i];
		for (int j = 0; j != i; j++) {
			if (s[i] == s[j]) {
				OK = 0;
				s[i] == 'N';
				s[j] == 'N';
				break;
			}
		}
		if (OK == 1 && c1 == 'N') {
			c1 = s[i];
		}
		else if (OK == 1 && c2 == 'N') {
			c2 = s[i];
		}
		else {
			OK = 1;
		}
	}
	if (c1 != 'N' && c2 != 'N') {
		cout << "Inpossible\n";
		return 0;
	}
	cout << c1 << '\n';

	return 0;
}