import java.util.Scanner;

// No.587 七対子
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String haipai = sc.next();
		StringBuilder machi = new StringBuilder();
		for(int i = 0; i < 13; i++) {
			String pai = haipai.substring(i, i + 1);
			if(count(pai, haipai) == 2) {
				continue;
			} else if(count(pai, haipai) == 1) {
				machi.append(pai);
				if(machi.length() != 1) {
					machi = new StringBuilder("Impossible");
					break;
				}
			} else {
				machi = new StringBuilder("Impossible");
				break;
			}
		}
		System.out.println(machi);
	}
	public static int count(String pai, String haipai) {
		int index = 0;
		int count = 0;
		while(true) {
			index = haipai.indexOf(pai, index) + 1;
			if(index == 0) {
				break;
			}
			count++;
		}
		return count;
	}
}