import java.io.*;
import java.util.*;


public class Main_yukicoder587 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		char[] s = sc.next().toCharArray();

		int[] cnt = new int[26];
		for (char c : s) {
			cnt[c - 'a']++;
		}

		int cnt1 = 0;
		int cnt2 = 0;
		char c = 0;
		for (int i = 0; i < 26; i++) {
			if (cnt[i] == 1) {
				cnt1++;
				c = (char)(i + 'a');
			} else if (cnt[i] == 2) {
				cnt2++;
			}
		}

		if (cnt2 == 6 && cnt1 == 1) {
			pr.println(c);
		} else {
			pr.println("Impossible");
		}
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);

		solve();

		pr.close();
		sc.close();
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}