import java.util.*;

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