/*
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 * and open the template in the editor.
 */

package lightswitch;

import java.io.File;
import java.io.FileNotFoundException;
import java.math.BigInteger;
import java.util.Arrays;
import java.util.Scanner;

/**
 *
 * @author ASUS
 */
public class LightSwitch {

    /**
     * @param args the command line arguments
     */
    public static char flip(char a){
        if (a=='0')return '1';
        return '0';
    }
    public static void main(String[] args) throws FileNotFoundException {
        Scanner sc=new Scanner(new File("out"));

//        Scanner sc=new Scanner(System.in);
        int tst=0;
        while (sc.hasNext()){
            tst++;
            int p1=0,p2=0;
            BigInteger bi=sc.nextBigInteger();
            char[] str1 = bi.toString(2).toCharArray();
            bi=sc.nextBigInteger();
            char[] str2 = bi.toString(2).toCharArray();
            int len=Math.max(str1.length,str2.length);
            StringBuilder str1b=new StringBuilder();
            for (int i=0;i<len-str1.length;++i){
                str1b.append("0");
            }
            str1b.append(str1);
//            System.out.println(str1b.toString());
            str1=str1b.toString().toCharArray();
            str1b=new StringBuilder();
            for (int i=0;i<len-str2.length;++i){
                str1b.append("0");
            }
            str1b.append(str2);
//            System.out.println(str1b.toString());
            str2=str1b.toString().toCharArray();
            if (str1.length==1){
                if (str1[0]==str2[0]){
                    if (str1[0]=='0')break;
                    System.out.print("Case Number "+tst+": ");

                    System.out.println(0);

                }
                else {
                    System.out.print("Case Number "+tst+": ");
                    System.out.println(1);
                }
                System.out.println();
                continue;
            }

            System.out.print("Case Number "+tst+": ");

            char[] str=Arrays.copyOf(str1, str1.length);
            int ans=1000000000;
            BigInteger moves=new BigInteger("-1");
            StringBuilder X=new StringBuilder("0");
            int sz=str.length;// , sol1=0 , sol2=1;

            for(int j=1;j<sz;j++){
                if(str[j-1]!=str2[j-1]){
                    str[j-1]=flip(str[j-1]);
                    str[j]=flip(str[j]);
                    if(j!=sz-1) str[j+1]=flip(str[j+1]);
//                    sol1++;

                    X.append("1");
                }
                else
                    X.append("0");
            }
            String s1=new String(str);
            String s2=new String(str2);
            if (s1.equals(s2)){
                p1=1;
//                ans=sol1;
                moves=new BigInteger(X.toString(),2);
//                System.out.println(moves);
            }
//

            //-------------------------------------------------------



            X=new StringBuilder("1");
            str=Arrays.copyOf(str1, str1.length);
            str[0]=flip(str[0]); str[1]=flip(str[1]);
            for(int j=1;j<sz;j++){
                if(str[j-1]!=str2[j-1]){
                    str[j-1]=flip(str[j-1]);
                    str[j]=flip(str[j]);
                    if(j!=sz-1) str[j+1]=flip(str[j+1]);
  //                  sol2++;

                    X.append("1");
                }
                else
                    X.append("0");
            }
     //       System.out.println(str);



            //-------------------------------------------------------
            s1=new String(str);
            s2=new String(str2);
            if (s1.equals(s2)){
                p2=1;
    //            if (sol2<ans){
//                    ans=sol2;
//                    moves=new BigInteger(X.toString(),2);
//                }
//                else if (sol2==ans){
                    BigInteger moves2=new BigInteger(X.toString(),2);
                    if (moves.equals(new BigInteger("-1"))||moves.compareTo(moves2)>0)
                        moves=moves2;
                //}
            }

            if (moves.equals(new BigInteger("-1"))){
                System.out.println("impossible");
            }
            else {
//                System.out.println(ans);
//                System.out.println(sol1+" " +p1);
//                System.out.println(sol2+" "+p2);
//
                System.out.println(moves);
            }

                System.out.println();
        }






        // TODO code application logic here
    }

}