import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

class Main{
	static Scanner s=new Scanner(System.in);
	public static void main(String[]$){
		Map<Integer,Integer> c=new HashMap<>();
		s.next().chars().forEach(i->c.merge(i,1,Integer::sum));

		int[] a=c.values().stream()
				.mapToInt(i->i)
				.sorted().toArray();
		if(Arrays.equals(a,new int[]{1,2,2,2,2,2,2})){
			System.out.println(
					(char)(int)
					c.entrySet().stream()
							.filter(o->o.getValue()==1)
							.findFirst().get().getKey());
		}else
			System.out.println("Impossible");
	}
}