import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws IOException {
		Scanner sc = new Scanner(System.in);
		String s = sc.next();
		HashMap<String, Integer> m = new HashMap<>();
		for (int i = 0; i < s.length(); i++) {
			String str = s.substring(i, i + 1);
			if (m.containsKey(str)) {
				m.put(str, m.get(str) + 1);
			} else {
				m.put(str, 1);
			}
		}
		boolean b = true;
		for (Map.Entry<String, Integer> entry : m.entrySet()) {
			if (entry.getValue().equals(new Integer(2))) {
				continue;
			} else if (entry.getValue().equals(new Integer(1))) {
				System.out.println(entry.getKey());
				b = false;
			}
		}
		if (b)
			System.out.println("Impossible");
	}
}