package net.ipipip0129.yukicoder.no587;

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        //文字列ソート処理
        List<String> piece_list = Arrays.asList(scan.nextLine().split(""));
        Collections.sort(piece_list);
        String piece = "";
        for (String tile : piece_list) piece += tile;

        // テキスト終了用 0x03 付与
        piece += String.valueOf((char) 0x03);

        scan.close();

        char missing = 0x00;
        boolean isError = false;

        while (piece.length() != 0 && !isError){
            char c_pair = 0x00;
            for (int i = 0; i < piece.length() ; i++){
                if (i == 0) c_pair = piece.charAt(i);
                else {
                    if (c_pair != piece.charAt(i)) {
                        if (i == 1){
                            if (missing == 0x00) {
                                missing = c_pair;
                            }else {
                                isError = true;
                            }
                        } else if(3 <= i){
                            isError = true;
                        }
                        piece = piece.replaceAll(String.valueOf(c_pair), "");
                        break;
                    }
                }
            }
            //残テキストが0x03のみなら終了
            if (piece.charAt(0) == 0x03) {
                break;
            }
        }

        if (isError || missing == 0x00) {
            System.out.println("Impossible");
        }else {
            System.out.println(missing);
        }
    }
}