import java.util.Scanner;

public class Main {
	static public void main(String[]args) throws Exception {
		Scanner sc = new Scanner(System.in);
		String s = sc.next();
		solve(s);
	}

	static void solve(String s) throws Exception {
		int [] alphabet = new int[26];
		for(int i=0; i<s.length(); i++) {
			int tmp = s.charAt(i) - 'a';
			alphabet[tmp]++;
		}

		int ans = searchAns(alphabet);
		if(ans == -1) {
			System.out.println("Impossible");
		} else {
			System.out.println( (char)('a' + ans) );
		}
	}

	static int searchAns(int [] alphabet) throws Exception {
		final int NUM_ALPHABET = 26;
		int count = -1;
		for(int i=0; i < NUM_ALPHABET; i++) {
			if(alphabet[i] == 0 || alphabet[i] == 2) {
				//nothing
			} else if(alphabet[i] == 1) {
				if(count  == -1) {
					count = i;
				} else {
					break;
				}
			} else {
				break;
			}
		}
		return count;
	}
}