import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		char[] arr = sc.next().toCharArray();
		int[] counts = new int[26];
		for (char c : arr) {
		    counts[c - 'a']++;
		}
		char ans = ' ';
		boolean flag = true;
		for (int i = 0; i < 26; i++) {
		    if (counts[i] == 0 || counts[i] == 2) {
		        continue;
		    } else if (flag && counts[i] == 1) {
		        flag = false;
		        ans = (char) (i + 'a');
		    } else {
		        System.out.println("Impossible");
		        return;
		    }
		}
	    System.out.println(ans);
	}
}