package m.shin;
import java.util.ArrayList;
import java.util.Scanner;
public class Con587 {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		String str = s.next();
		s.close();

		ArrayList<String> delete = new ArrayList<>();
		boolean ft = false;
		String ans = "";
		for(int i = 0;i < 7;i++){
			String c = str.substring(0,1);
			str = str.substring(1, str.length());
			if(delete.contains(c)){
				break;
			}else{
				delete.add(c);
				if(str.indexOf(c) == -1 && !ft){
					ft = true;
					ans = c;
					continue;
				}else if(str.indexOf(c) == -1 && ft){
					ft = false;
					break;
				}else{
					str = str.replaceFirst(c, "");
				}
			}
		}

		if(ft){
			System.out.println(ans);
		}else{
			System.out.println("Impossible");
		}

	}

}