import java.util.*;

public class paiza {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String str = sc.nextLine();
		int count = 0;
		String match = "";
		char ch;
		
		for (int i = 0; i < str.length(); i++) {
			ch = str.charAt(i);
			if ( str.indexOf(ch,i+1) != -1) {
				if ( match.indexOf(ch) == -1 ){
					count++;
					match = match + ch;
				}
			}
		}
		
		if (count == 6) {
			for(int i=0; i<str.length(); i++) {
				if(match.indexOf(str.charAt(i)) == -1){
					System.out.println(str.charAt(i));
					return;
				}
			}
			System.out.println("Impossible");
		}
		else {
			System.out.println("Impossible");
		}
	}
}