import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;
import java.util.HashMap;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            HashMap<Character, Integer> h = new HashMap<>();
            String s = in.next();
            for (int i = 0; i < 13; i++) {
                char c = s.charAt(i);
                h.put(c, h.getOrDefault(c, 0) + 1);
            }
            String ans = "Impossible";
            for (char c : h.keySet()) {
                int v = h.get(c);
                if (v == 1 && ans.equals("Impossible")) ans = String.valueOf(c);
                else if (v != 2) {
                    ans = "Impossible";
                    break;
                }
            }
            out.println(ans);
        }

    }
}